Here is the stat block I made for Venger:
Venger, Male Half-Fiend Human Sor18: CR 21; M Outsider [Native]; HD 18d4+54; hp 100; Init +3; Spd 30 ft., fly 30 ft. (average); AC 21 (+3 Dex, +1 natural, +7 leather armor), touch 13, flat-footed 18; BAB/Grap +9/+11; Atk +11 melee (1d4+2, claw); Full Atk +11 melee (1d4+2 [x2], claws), +6 melee (1d6+1, bite); SA smite good (1/day, +18 damage vs. good opponent), spell-like abilities; SQ darkvision (60 ft.), DR (10/magic), immunity (poison), resistances (acid, cold, electricity, and fire 10); SR 28; AL LE; SV Fort +9, Ref +8, Will +14; Str 14, Dex 15, Con 16, Int 15, Wis 12, Cha 21.
Skills: Bluff +18, Concentration +24, Craft (alchemy) +13, Diplomacy +7, Intimidate +9, Knowledge (arcana) +23, Knowledge (the Realm) +12, Spellcraft +25, Spot +3; Feats: Combat Casting, Dodge, Empower Spell, Eschew Materials, Iron Will, Heighten Spell, Maximize Spell, Spell Penetration.
SA—Spell-Like Abilities (Sp): 3/day—darkness, poison, unholy aura; 1/day—blasphemy, contagion, desecrate, horrid wilting, summon monster IX (fiends only), unholy blight, unhallow.
Arcane Spells Per Day (6/8/7/7/7/7/6/6/5/3; save DC 15 + spell level).
Arcane Spells Known (9/5/5/4/4/4/3/3/2/1).
Possessions: Leather armor +5.
You are all, of course, free to disagree with this stat block in any way you see fit. lol
I didn't select his spells known, since I felt that it would be easier for a DM to customize him that way. Give him whatever spells you want or need him to have.
As for his backstory, someone already mentioned the Requiem story, which is fairly easy to find with a Google-search.