Here you go SJ. This is a character you can throw in as an encounter that will give your players a good run for their money. For all of those in the forum do note this a character made using 3.0 Rules and nothing more than the 3.0 Player's Handbook, Monster Manual, and Dungeon Master's Guide.
(Note Attributes were taken from the stats for a Level 1 NPC Fighter in the DMG, and then Half Fiend Template, Plus Levels and Magic Item Effects. Magic items were based on the DMG chart for determining gold based on level.)
Human Half-Fiend Fighter
Level: 12 (3 for Half-Fiend, 9 for Fighter)
Hit Points: 9d10 (81 Hps Avg)
STR: 22 (+6), DEX: 18 (+4), CON: 16 (+3), INT: 14 (+2), WIS: 12 (+1), CHA: 10 (+0)
FORTITUDE: 12, REFLEX: 10, WILLPOWER: 7
BASE ATTACK BONUS: +9/+4
AC: 26, TOUCH AC: 16, FLAT FOOTED AC: 22
SQ: Darkvision 60', Immune to Poison, Acid/Cold/Electricity/Fire Resistance: 20, Darkness 3x/day, Desecrate 1x/day, Unholy Blight 1x/day, Poison 3x/day, Contagion 1x/day
FULL ATTACK (Natural): Claw/Claw: +10/+10, 1d4 + 3/1d4 + 3, Bite: +15, 1d6 + 9
FULL ATTACK (Spiked Chain, Main Hand): +13/+8, 2d4 + 7
FULL ATTACK (Spiked Chain, Off - Hand): +13/+8, 2d4 + 4
STANDARD ATTACK (Spiked Chain): +17, 2d4 + 10
FEATS: Exotic Weapon Proficiency (Spiked Chain), Expertise, Combat Reflexes, Improved Disarm, Improved Trip, Ambidexterity, Two-Weapon Fighting, Blind Fighting, Improved Two-Weapon Fighting, Weapon Focus (Spiked Chain)
Weapon: +1 Wounding Spiked Chain
Armor: +2 Light Fortification Chain Shirt
Shield: +1 Animated Large Steel Shield
Hands: Guantlets of Ogre Power
Fingers: Ring of Protection +2, Ring of the Ram (w/50 Charges)
Feet: Slippers of Spider Climbing
Neck: Necklace of Fireballs Type VI
Back: Cloak of Resistance +3
Quiver: Javelin of Lightning x5
Carried: Potion of Blur, Potion of Haste x2, Potion of Enlarge, Potion of Cure Serious Wounds (X4), Potion of Cat's Grace, Potion of Aid, Oil of Slipperiness (x2), Tangle Foots Bags (x4), Thunderstone (x4), 30 Gold
Tips for Playing
Play this character intelligently as he does have a higher than average INT for a fighter.
Use the Oil of Slipperiness on himself and Potion of Cat's Grace before the encounter takes place.
Have him use the slippers for running up walls and other sorts of barriers and use his reach with the chain to attack foes from those positions. Since he will most likely be on higher ground this gives him a +1 Bonus to Attack.
Use Thunderstones on any Arcane Casters and Tangle Foot Bags on Divine Casters.
Target your Rogues or other high dexterity based characters with Disarms/Trips.
Target the Fighter types or those Strength types with the necklace of fireballs. Or even better have the area they are fighting coated with the extra Oil of Slipperiness.
Trick to this guy is keep him at range from the party. Keep him mobile and you should give the party a run for their money.