Ulgoroth, Herald of Brimstone, who took Kaelgard in YD 162 and raised it higher in black glass. He bears the Brimstone Key and cannot use it, and it burns him; and he will not let it go. He summons fire demons, rages when wounded, opens rifts beneath your feet, and drags you to him.
Stats
Level
46 (data base level 46; monsters are rescaled to the level of the spawner they come from)
Hit points
31,068 at level 46
Damage
37–69 per hit (×14.02 in combat before your armour: ≈ 519–968)
Damage type
Fire
Armor class (defence)
112 (blocks that share of your weapon damage: AC / (AC + 100 + 8 × your level), max 60%; spells ignore AC)
Accuracy (hit)
62 (its chance to hit you = 80% + 1% × (accuracy − your AC), 10–95%)
Attack speed
one attack every 1.5 s
Move speed
45 (game units; higher is faster)
XP when killed
8,486 at level 46
Element
Fire
Family
Demon
Stats by spawn level
Level
HP
Damage
Hit (×combat mult)
AC
Accuracy
XP
46
31,068
37–69
519–968
112
62
8,486
Resistances & weaknesses
Fire
Ice
Poison
Earth
Storm
Holy
Dark
−25% damage
takes +50%
normal
normal
normal
normal
normal
Derived exactly as the game does (elemental_mult): same element ×0.75, opposite element ×1.5, Void creatures ×0.8 to everything. Physical damage is reduced by AC instead.
Any Unique item(a random unique, from any monster)
0.10%
Unique
Loot table boss_ulgoroth: 1 roll per kill. Equipment tier follows monster level (level 46 → tier 8, Hellstone / Frostforged; ±1 tier 10% of the time). Champions roll one extra gear roll with ×4 gear chance. Chances are the in-game Bestiary rates (Bestiary.drop_table); set pieces and uniques are the global 1% / 0.10% rates.
Brimstone Heart ×3, 
Demon Horn ×4.
Greater Health Potion
Hellstone Ore
Drake Steak
The Brimstone Key
Brimstone Ring
Hellforged Greaves
Brand of Ulgoroth