From what I could research, there are only 3 bosses besides from the Gaz'Haragoth, that drops umbral items (honestly I thought that only Gaz dropped these items):
The Last Lore Keeper - who drops Crude Umbral Hammer and belongs to the Forgotten Knowledge Quest. (Final Boss)
The Source Of Corruption - who drops Crude Umbral Slayer & Umbral Slayer it belongs to the Cults of Tibia Quest. (Final Boss)
Lady Tenebris - Crude Umbral Spellbook & Umbral Spellbook, who also belongs to the Forgotten Knowledge Quest. (Mini boss)
and of course...
Gaz'Haragoth who drops: Crude Umbral Axe, Crude Umbral Blade, Crude Umbral Bow, Crude Umbral Chopper, Crude Umbral Crossbow, Crude Umbral Hammer, Crude Umbral Mace, Crude Umbral Slayer, Crude Umbral Spellbook, Umbral Blade, Umbral Bow, Umbral Chopper, Umbral Crossbow, Umbral Hammer, Umbral Mace, Umbral Slayer, Umbral Spellbook, Umbral Master Axe, Umbral Master Bow, Umbral Master Chopper, Umbral Master Crossbow, Umbral Master Hammer, Umbral Master Mace, Umbral Master Slayer, Umbral Master Spellbook, Umbral Masterblade.
Source: https://www.tibiawiki.com.br/wiki/Gaz%27Haragoth
"Gaz'haragoth is a Demon Prince who was once in charge of overseeing Roshamuul, until the explosion of the Dream Catcher mutated him into his current form.
After a world breaks Roshamuul's wall, Gaz'haragoth is found on top of the mountain at the center of the island, and will remain there until it's defeated. Upon defeat, Gaz'haragoth is cast to the Roshamuul Prison lowest levels, where it will be imprisoned in a secure room. Even so, eventually he gathers considerable strength and can be fought again by brave warriors.
Before he appears, the following message is broadcast in white text to all players of the world he spawns in:
Gaz'haragoth will shatter your dreams in a barrage of nightmares!"
Source: https://tibia.fandom.com/wiki/Gaz%27haragoth
Hope this helps you and clarify some of your questioins, happy new year!