How did I get the 928 hit point poison is Skyrim

The quest that gave you infinite magicka regeneration is Out of Balance

From the UESP site:

When cleaning the foci, a bugged two hour buff may be placed on you that increases magicka regeneration immensely. It is possible to get multiples of this buff during the quest. In the active effects category, it appears as "Fortify Magicka". Selecting it reveals it as "Drain Magicka, +25 Fortify Magicka for 2 hours". It doesn't appear to increase magicka capacity at all (as listed on the skills screen), but increases magicka regeneration rate instead.

Some players have reported that the buff remains much longer than the stated 2 hours. In this case, the buff(s) can be removed by opening the character editor with the "showracemenu" console command (on PC) and closing it again. This will also remove other temporary buffs, such as blessings, and enchanted apparel will need to be re-equipped to restore their effects. (v1.1.21.0 of game)

The reason why your poison is so high is because you used the jarrin root. It is extremely deadly and I've seen potions with it that can go much higher. The wiki says that jarrin root and deathbell can make a poison of 2400 damage.

You should have saved it as there's no way to obtain a second jarrin root except through the console.

Through normally obtainable ingredients, the highest damage poison I was able to make (while wearing 5 pieces of fortify alchemy gear, including usage of the falmer helmet glitch, all necessary alchemy perks and alchemy skill at 100) was this:

This was done with these ingredients:

• Crimson Nirnroot
• Deathbell
• Small Antlers

For some reason, crimson nirnroot makes the strongest damage health effect @221, all other combinations do not come close.

Assuming the target has no poison resistance, this will end up being roughly 384 instant damage. Here are combinations of damage health with other effects:

Crimson Nirnroot + Giant Lichen + Jazbay Grapes:
The ravage health comes handy since it lowers the max hp of a target so they cannot heal themselves to regain that hp.

Crimson Nirnroot + Imp Stool + Slaughterfish Eggs:

If you do not care about instant damage, this is probably the highest damage poison you can make in the game with over 500 poison damage.

And finally, for a fun experiment, here is what my maxed out alchemist can make with a jarrin root:
Roughly 12.8k damage(take that Alduin!)