Let's start with the seeker. Here's what we know about it from the game:
Str 10, Dex 10, Con 10, Int 13, Wis 13, Cha 10
AC 2
40 HP
No resistances
Eye rays: Aganazzar's scorcher, flame arrow, Melf's acid arrow, magic missile.
We'll use Krishnath's suggested HD (5d8).
Here's the stat block:
Beholderkin, Seeker
Small Aberration
Hit Dice: 5d8+10 (33 hp)
Initiative: +6
Speed: 5 ft. (1 square), fly 20 ft. (good)
Armor Class: 18 (+1 size, +2 Dex, +5 natural), touch 13, flat-footed 16
Base Attack/Grapple: +3/-3
Attack: Eye rays +6 ranged touch and bite -3 melee (1d4-2)
Full Attack: Eye rays +6 ranged touch and bite -3 melee (1d4-2)
Space/Reach: 5 ft./5 ft.
Special Attacks: Eye rays
Special Qualities: All-around vision, darkvision 60 ft., flight
Saves: Fort +3, Ref +3, Will +7
Abilities: Str 6, Dex 14, Con 14, Int 13, Wis 13, Cha 11
Skills: Hide +14, Knowledge (arcana) +9, Listen +3, Search +9, Spot +11, Survival +1 (+3 following tracks)
Feats: Alertness (B), Improved Initiative, Iron Will
Environment: Cold hills and underground
Organization: Solitary, pair, or cluster (3-6)
Challenge Rating: 4
Treasure: Standard
Alignment: Usually lawful evil
Advancement: 6-10 HD (Medium); 11-15 HD (Large)
Level Adjustment: --
The creature is small and spherical, and seems to float effortlessly in the air. It has a bulbous body with a central, unblinking eye, and a large maw filled with needlelike teeth. Four smaller eyes, attached to wriggling stalks, sprout from the top of the orblike body.
Seekers are mutant beholderkin created by Lycanth the Enlightented, a powerful wizard in the service of the dragon Abazigal. Because seekers are created via experimentation on standard beholderkin, true beholders and their kin will attack them on sight.
A seeker is a 3-foot wide orb dominated by a central eye. Four smaller eyes on stalks sprout from the top of its body. Unlike many beholderkin, its central eye serves no special function.
Seekers speak the language of Beholders and Common.
Combat
The primary weapon of the seeker is a series of deadly eye rays.
Eye Rays (Su): Each of a seeker's four eye rays resembles a spell cast by a 7th-level caster. Each eye ray has a range of 100 feet and a save DC of 12. The save DCs are Charisma-based. The four eye rays inclue:
Magic Missle: This works like the spell, firing four missles, each causing 1d4+1 points of damage (no save).
Melf's Acid Arrow: This works like the spell, causing 2d4 acid damage for two rounds (no save).
Scorching Ray: This works like the spell, causing 4d6 points of fire damage (no save). The seeker may fire two scorching rays per round.
Searing Light: This works like the spell, dealing 3d8 points of damage to living creatures, 7d6 vs. undead, 7d8 vs. light-sensitive undead, and 3d6 vs. constructs and inanimate objects (no save).
All-Around Vision (Ex): Seekers are exceptionally alert and circumspect. Their many eyes give them a +4 racial bonus on Spot and Search checks, and they can't be flanked.
Flight (Ex): A seeker's body is naturally buoyant. This buoyancy allows it to fly at a speed of 20 feet. This buoyancy also grants it a permanent feather fall effect (as the spell) with personal range.
The seeker first appeared in the Black Isle computer game Baldurs Gate II: Throne of Bhaal, (2001).