I registered here solely to shed some light on how to get knowledge of this guy. I have tried lots of stuff, watched videos on it, got all the Amity mentioned here, did quests at the Western Guard Camp, nothing worked. The eventual solution was the following:

* You need to play the full main questline with a character, you cannot get it when doing the simplified main questline.

* When doing the main questline the first quest in the category "[I] The Young Lion of Heidel" is a crossroad quest and you need to select the LEFT one, which is called "The Lamenting Head of the League of Merchants" ( https://bdocodex.com/th/quest/21004/1/ ).

* Once you have selected this quest line you have to play through all the quests up until the quest category "[I] Trapped", where the second to last quest is called "Confronting Jordine" ( https://bdocodex.com/th/quest/21011/5/ ).

* The completion target for that quest is Crucio Domongatt and after you hand it in you can chat with him and get his knowledge.

All that being said, as mentioned, I have done all the quests at the Western Guard Camp, which start with Jarette Domongatt, with whom you have to get 501 Amity in order to get the quest "I Care About You" ( https://bdocodex.com/th/quest/1067/1/ ). This is the first quest in a small questline that might be necessary to get knowledge of Crucio Domongatt.
However, since I have done those quests before I did the whole crossroad main questline I do not know whether or not these quests (or the Amity with Carolin, for that matter) are actually needed to get the knowledge on Crucio Domongatt.

By the way, selecting said crossroad main questline and playing through the mentioned quests is also the solution on how to get the knowledge of "Southern Naga Marsh" ( https://bdocodex.com/th/theme/3072/ ) and "Central Naga Marsh" ( https://bdocodex.com/th/theme/3069/ ) from the NPC Kanna ( https://bdocodex.com/th/npc/41076/ ) in Glish. After the quest "Donatt, the Forgotten Name" ( https://bdocodex.com/th/quest/21008/3/ ) you can talk to Kanna to get knowledge of the Southern Naga Marsh and then you can talk to her again to get knowledge of the Central Naga Marsh.

need 201 amity with carolin

from what I can tell, you can only gain knowledge Crucio Domongatt once you have to completed all quests from Jordine Ducas

i think i did everything but i cant learn crucio domongatt knowledge why pls help me

Wish someone would post where he actually is :/