I found that when you mix a Human heart, Daedric Heart, and a Jannin root it makes a poison that does 3000 damages to health, 100 magic regeneration damage, and 45 magic damage.

That would be cause of the jannin root

My favorite is a simple little concoction made from Canis Root, Imp Stool, and Mora Tapinella. It's a Lingering Damage Health and Paralysis poison. It damages health as they lie paralyzed. Definitely not the strongest poison, but I admire its nefarious nature.

That is just insult to injury! Devious! (I love it.)

It's very useful. If it suits the character I'm playing, I usually carry a few around me for an edge in difficult encounters.

Strongest poison is probably Crimson Nirn Root, Skeever tail and Scathecraw.