Wow! Thanks everyone - these are great. Attached is my version so far, subject to tweaks. Want to kibitz and dissect it? Feel free! Advice, both positive and negative, is welcome. It's an amalgam of all your ideas, combined with Hellhound's excellent Channeller Knight prestige class. The concept is "give lots of choices, but limit power to what's appropriate for a high lvl game." Any help fine-tuning it would be great.
Note that the attack progression is dropped to a cleric's. My gut feel is that this class as written won't unbalance my game (the PCs taking it are 16th and 17th lvl).
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Radiant Knight of Aeos
It is rare for the God of the Sun and the forge to directly address his worshippers. When he does, however, his presence may alter the very nature of the champions who bask in his divine radiance. Thus are radiant knights created. There have been five known sets in the history of the religion, rare enough that theologians are known to debate whether or not their very existence is a myth swollen by fable and time.
Radiant knights always appear in pairs. Their powers often do not appear for months after first viewing the avatar of their God, and are always heralded by an itching spot of freckled skin in the shape of a sun (often referred to as a “sunspot,” a common nickname for the class.) There is a duality to radiant knights; one is always drawn to the dawn and one to the dusk, and certain of their powers can only be used by acting in concert. Radiant knights of dawn are often drawn to powers of brash combat, and radiant knights of dusk are often drawn to powers of divination and defense. This is by no means consistent, however.
As their bodies resonate with the divine radiance of their God, radiant knights learn to focus positive energy, using their prayers as a lens which can then focus the divine light into a variety of different uses. No two radiant knights gain identical powers, but all become mighty servants in the name of their god.
Only once has a radiant knight fallen from grace, using her channeling powers for selfish purposes as she was hunted by the militias of her church. Her name has been stricken from the Book of the Sun, and the location of her tomb has been forgotten.
Class Features
Hit Die: d10
Requirements
To qualify to become a Radiant Knight of Aeos (either Dawn or Dusk focused) a character must fulfill all the following criteria.
Base Attack Bonus: 12+
Alignment: Any Good
Feats: Martial Weapon Proficiency (any 2), Shield Proficiency, Heavy Armour Proficiency, Weapon Focus (1 martial weapon).
Undead Turning: 8+ per day
Special: Blessing of the church of Aeos. Must have been exposed to the divine radiance of Aeos’ avatar. Must have been healed of major damage (at least 75% of total hit points) by divine curing magic.
Class Skills
The Radiant Knight's class skills (and the key ability for each skill) are Concentration (Con), Diplomacy (Cha), Heal (Wis), Intimidation (Cha), Jump (Str), Knowledge (Religion) (Int), Profession (Wis) Ride (Dex), and Sense Motive (Wis).
Skill Points per level: 2 + Int modifier
All the following are class features of the Radiant Knight prestige class.
Weapon and Armor Proficiency: Ascension to the Radiant Knight class does not grant the character additional Weapon and Armor Proficiencies.
Note: The radiant knight class does not stack with other classes in terms of spellcasting.
Class Level  Attack Bonus Fort Save Ref Save Will Save Special
1 +0 +2 +0 +0 Positive energy mastery, Channel the Sun I
2 +1 +3 +0 +0 Channel the Sun I, Mingle the Light
3 +2 +3 +1 +1 Channel the Sun II
4 +3 +4 +1 +1 Channel the Sun II
5 +3 +4 +1 +1 Channel the Sun III
Flavor abilities (Su): During the daytime the equipment, eyes, teeth and hair of a radiant knight seem to gleam and shine in an attractive manner. Radiant knights cast no shadows caused by the sun, and they involuntarily cause others nearby to tan. Flowers brighten and subtly turn towards a radiant knight, and cats tend to bask in their presence. Radiant knights who identify with the dawn always knows when it is sunrise; radiant knights who identify with the dusk always knows when it is sunset.
These flavor abilities have no in-game effect.
Positive energy mastery (Su): Radiant Knight levels stack with all other classes for the purpose of determining the character’s ability to turn undead. Furthermore, the radiant knight gains one extra turning attempt per class level per day.
Channel the sun (Su): The Radiant Knight learns many ways to channel the positive energy that flows through them, the least of these being the turning of the undead. At each level indicated, the Radiant Knight can choose one power from the category listed or from a category of lower level than the one listed. Using a channel is a quickened action for a Radiant Knight, and each use counts as an attempt to turn undead (thus reducing the number of turnings the character has left that day). If a Radiant Knight can no longer turn undead that day, he can use no further Channels. A Radiant Knight can only perform one channel per round.
In lieu of a standard gained feat, a radiant knight may select one additional Channel the Sun class ability of any level he or she qualifies for.
Channel the sun I (select one of the following)
Aura of Purity: You and all your allies within 30 feet get a +4 sacred bonus on all Fortitude saves for 2 rounds per level of Radiant Knight and all secondary effects that have not yet occurred of poisons already in effect are cancelled (as a remove poison spell).
Breach the Shadow: For one round per point of charisma bonus, you may make a weapon that you have the weapon focus feat for into a ghost touch weapon.
Bring the Light: For ten minutes per class level after activation, a flash of sunlight bursts from the point of impact when your weapons strike an opponent. Any creature struck which is vulnerable to sunlight must make a fortitude save (DC 10 + Charisma modifier + Radiant Knight level) or suffer the effect of sunlight (i.e. vampires are burned, and drow suffer penalties to attack.)
Eyes of the Sun: (Sp) Your eyes can emit searing light (as per the spell) as your next standard action. The caster level is considered your effective turning level.
Healing Light: You and all your allies within 10 feet are cured of damage equal to 1d8 + 1 per level of Radiant Knight + Charisma Bonus (max of 1d8+15).
Sacred shield: You and your allies within 15 feet each gain a +2 sacred bonus to AC for one round plus one round per point of Charisma bonus (if any)
Sense of Corruption: Your ability to detect evil and/or undead permanently becomes a 360o radius emanation instead of a standard cone.
Shining Blade: You gain a +1 to the critical threat range of a weapon you have the Weapon Focus feat for, for one round per point of charisma bonus.
Smite of Aeos: Your next attack deals +1d6 damage and the opponent must make a Fort save (DC 10 + Radiant Knight Level + Strength Bonus) or be knocked back 10 feet from the impact – if the target cannot move back that far it falls prone as far back as it can be moved and takes an additional 1d6 damage.
Solar Resistance: You and all allies within 30 feet gain immunity to blinding (by bright light or spells such as color spray) and resistance to fire of 12 points for 1 round per Radiant Knight level. This resistance does not stack with similar resistances, such as those granted by spells or special abilities. In addition to this activated effect, you are continually unaffected by bright lights, and can stare directly into the sun. You are always immune to negative physical effects of the sun such as sunburn, sun stroke, and sun-related skin diseases. This ability grants a +10 bonus to Wilderness Lore and Fortitude saving throws when trying to survive in a desert environment.
Sunlit Vigor: You gain a burst of speed and vigor, increasing your base speed by 10 feet and giving you a +2 sacred bonus to your Constitution for 2 rounds plus twice your Charisma modifier (if any).
Channel the Sun II (select one of the following, or one choice from Channel the sun I)
Blessed Aim: You and all your allies within 10 feet get a +4 sacred bonus on all attacks this round, as beams of sunlight illuminate your enemies’ vulnerable spots.
Righteous Charge of the Dawn: Your next charge attack (which must occur within one round per class level) is imbued with the strength of the sun. Charge distance is up to twice normal, you and your mount (if any) receive your charisma bonus as a dodge bonus vs. AoOs during the charge, and the opponent struck at the end of the charge receives double damage.
Forge of Aeos: Your chosen weapon is temporarily imbued with the essence of Aeos’ forge. If using a weapon for which you possess the feat weapon focus, all attacks for one round deal an extra +3d6 holy damage vs undead. This does not stack with other holy damage from a holy weapon.
Incandescent Strength: You and all your allies within 10 feet add a sacred bonus to Strength equal to your Charisma bonus (minimum 2) for 1 round per point of Charisma bonus.
Radiant Blade: You gain a +1 to the critical multiplier of a weapon you have the Weapon Focus feat for, for one round per level of Radiant Knight.
Revealing Light: Aeos grants the ability to pierce any darkness, and - at times - even the ability to see through most cunning magical deceptions. For one hour per Radiant Knight level, you become immune to all forms of darkness - magical or otherwise - seeing at all times as if in the bright light of day (physical barriers still block your vision, however). Additionally, incorporeal, invisible, and similarly concealed creatures are revealed to you, and you are aware that such creatures are attempting to use concealing magic. Mundane hiding and concealment are still effective.
Channel the Sun III (select one of the following, or one choice from Channel the Sun II, or two choices from Channel the Sun I)
Angelic form: As your next standard action you can cast the spell Greater Aspect of the Deity (DotF), with caster level equal to your effective turning level.
Aura of Renewal: You may grant all allies within a 15’ radius (including yourself) fast healing 5 while within range of the aura. This ability lasts for a number of rounds equal to your charisma bonus.
Holy Fire: You are wreathed in a halo of white flames for 1 round per Radiant Knight level. Any evil enemies and undead within 5 feet of you take 1d8 damage plus your Charisma bonus (if any) per round spent within this aura.
Smite the infidel: Your attacks with a weapon you have the Weapon Focus Feat for deal an additional 2d6 damage against evil enemies and undead for one round per level of Radiant Knight as it burns with holy fire.
Zealous Destruction: Your next attack action can be done as if you had the Whirlwind Attack feat.
Mingle the Light: In order to use any Mingle the Light ability, both Radiant Knights must be within 10’ of one another.
Sear the Unclean (Su): When assisting a turning attempt by another radiant knight, you may grant a +4 bonus instead of the normal +2 bonus.
Purification (Sp): By each spending three turning attempts, two Radiant Knights praying together may channel the pure light of Aeos into the afflicted body of another. The recipient is immediately purged of all diseases or poisons, and is additionally subject to the effects of a restoration. Purification requires a full-round action by both characters to use, and provokes an attack of opportunity.
Radiant Glory (Sp): By each spending three turning attempts and each taking a standard action, two Radiant Knights praying together may replicate the spell sunburst, with a caster level equal to the average of each knight’s effective turning level.
Rekindle the Flame (Sp): By each spending eight turning attempts and each taking a full round action, two Radiant Knights praying together may replicate a True Resurrection spell upon anyone who has been dead less than 3 rounds.
Sunshower (Sp): By each spending two turning attempts, two Radiant Knights praying together may summon lantern archons. A beam of true sunlight (from the sun or some magic which creates real sunlight) must be visible. The archons slide down the sunbeam and arrive as the full round prayer ends, and remain in the area for one round per total class level of the two Radiant Knights. One lantern archon appears for every point of the two radiant knights’ total charisma bonuses. The lantern archons are free-willed, but are predisposed to be friendly to the summoners, and will generally attack the summoners’ enemies if not requested to perform other actions. Sunshower requires a full-round action by both characters to use, and provokes an attack of opportunity.