Damn I was looking to make a Damien guide, but your's looks really slick. There does seem to be some small mis-info, so I'll try to point them out and hopefully we can make your guide into the best there is!

1. At least in Normal Damien, stigma stacks do no reset on death. Here's a Damien vid showcasing this.
2. From that time stamp, you can also see that the sword pulse only does 90% Max HP damage.
3. You should also note that all of Damien's attacks add a stigma stack, not just the Divebomb. People most frequently get stigma stacks from his Nebula Strike (both phase 1 and 2).
4. The Transcendent Orbs reduce damage and healing to 10%. I'm pretty sure you already know that and it's just a typo.
5. For Damien's Root Trap, make sure to include that if you don't break out in time and hit him enough to knock him into the ground, he will completely fucking destroy everybody.

There's a few more things I'd maybe like you to add in the future (hopefully I'm not being a pest >.>), but I really like the way your guide looks! Especially with all the GIFs of his attacks. Also thank god you listed his attack names because the names I used for them were so dumb

You guide has great info on 'knowing' Damien, so I'll probably give you some things to add for 'fighting' Damien. But for now, I hope you update the things I listed, and great guide!