So, I tested it in Ultra Greed and Ultra Greedier on my 2016 mid range laptop. I gave me Summoning Dark via console as well as infinite HP and quick kill ability till right before the boss fight. The game didn't crash for me and didn't even slow down.

Thanks for testing it and for the mini dump :) I haven't touched or even used this mod in a long time, but this might be a good opportunity to do so again.

On the first playthrough I encountered the issue on I had quite a few mods, but I tried debug runs with only the sandbox mod to get to the Ultra Greed fight and it's consistent - even with only this mod and the sandbox mod (Da Rules), or with only this mod (removing Sandbox just before entering the Ultra Greed boss room), the game crashes as soon as Ultra Greed begins stomping on the ground, just before Keepers would be summoned (sometimes he does this at the start, sometimes he doesn't and it's all smooth until he does it). I don't know if it's any help, but there's also several minidumps (that I don't know how to read) and text logs, which seem to end mostly the same way:
https://pastebin.com/vuhHvRm1
I hope this helps, though mind you it's not the biggest issue, the mod works great through the normal game as far as I've seen and only has issues during this specific boss fight.

Thanks for letting me know. I will take a look soon. Do you have have any other mods installed?

I have no idea of why or how, but this mod seems to consistently cause the game to crash when Ultra Greed does his stomp attack that summons Keepers (Repentance)

ahoron cool item :3 but erfly is a douch he problaby just mocking you

Sure thing. ;D
( I just thought, Darkness = Satan and all. ;P) - (afterall it's binding of isaac. Haha!)

Or hey! I have another idea! Is there anything in Discworld Novels with the Summoning Dark about a dark roaming spirits?!

Cause, I just imagined in my head that a dark spirits (imagine a dark evil version of Mr. Me ghosty goo) starts to spawn (max to 3) roam around and damage enemies (damage scales with the "Isaac's" damage) with chance of inflitcing fear status (scales with luck).

Tick damage will be something like Cube Of Meat does.

...or something like that. XD

Haha, thanks for the feedback and the idea! I think the Summoning Dark isn't related to a Satan-like entity in the Discworld novels, so that I don't want to relate it just to "Evil Up" items. But I might take a look on Darkness-related items in the game and how rare they are.
Originally it was just an idea I had and wanted to implement, but now I might update it. Thanks again!

Hmm... cool idea, but I feel like it could be a bit better, cause quite frankly, I wouldn't pick up an item that gives me a perma-curse of darkness for a small damage up and teensy speed up.

I think adding a small buff to this could be nice... In what form?! Well.....

Maybe, every item with "EVIL UP" thing could give an extra +1.00 damage and if Brimstone is collected, it gives a special buff of +2.00 damage (since it's rare and it is a signature item of Satan.)

It's just an idea and my silly opinion, so don't take it too seriously. ;P

@NAR Bob: Yes it is :) Wanted to change the known phrase to a more "demonically" meaning

You got some really good ideas I gotta say :) I guess I'll have to raise the brightness of my screen xD

@The Staer: Thanks for the feedback! I'm working on something. I will test the item with increased damage (to 2.0 or more? Will see), but will decrease the damage for every burning fireplace in the room. Will take some days to be implemented and to be sure if this might improve the item ;)

I think it's a little underpowered, because that curse is really annoying :/

@Chara: I'm trying my best ;) The modding API and I are not the best friends yet though...

@Erfly: Thanks! "The Rose" had one of the scripts I used as reference ;)

@One Man Biker Gang: It goes baa, that is a sheep!

This is not my cow.

Looks very nice!

please oh please do the trinket thing