First of all congratz..
i noticed you received the codex entry: Dreadtooth (frenzied).
I have only done Dreadtooth on 1 and 2 stacks and just received a normal Dreadtooth codex entry.
At which stacks do you get the other codex entries?
Could it be:
1 stack => normal
3 stacks => corrupted
5 stacks => frenzied
7 stacks => nightmarish
Got the codex entries from torhead.com but it doesn't say how to get them.
I can confirm on the 1, 3 , and 5 stacks as I now have them all. I'll let you know on the 7 stack if/when we do it ;p