What you were expecting to happen, and what actually happened?
Tzathoth’s skill says 7 damage and 20% devour chance but so far it’s been 100% devour over 10 tries.
What are the steps to make it happen again?
Fight Tzathoth and have him use his skill.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!