Upper_Krust
Legend
Hey Fieari mate! 
I like it.
That last bits a tad unusual.
Makes sense.
Presumably.
Ouch!
Full damage compared to the Eldest Dragons of each colour? Or full damage compared to a Level 35 Solo Challenge?
Still nothing to solve the stun problem though?
Okay.
Might be interesting as a PC ability, give them the choice to flee (and fight another day) or stay and risk their immortal existence.
Surely they mean anyone Level 20 or lower? Epic Tier is 21-30 after all.
Its easy to use the higher of Artifact Level or Character Level for this type of thing.
Seems a no-brainer, but good that they included it.

Fieari said:Tiamat gets a standard + minor action per head,
I like it.
and each head gets a separate initiative, prerolled by the statblock even!
That last bits a tad unusual.
Each head also gets a separate action point.
Makes sense.
I'm not sure how movement is handled. I guess any head can forgo its standard for a move action.
Presumably.
There's also an additional ability that when all heads have acted, she can use a combined attack.
Ouch!
It definitely looks to me like each head does full damage.
Full damage compared to the Eldest Dragons of each colour? Or full damage compared to a Level 35 Solo Challenge?
It also gives a standard template for all deities. They always get a saving throw against effects and ongoing damage, even if the ability says otherwise.
Still nothing to solve the stun problem though?
They auto-know supernal.
Okay.
They auto-escape at bloodied (so that's not just a tiamat thing as was implied by Ltheb).
Might be interesting as a PC ability, give them the choice to flee (and fight another day) or stay and risk their immortal existence.
They can't be effected by anyone level 19 or lower.
I'm not sure I like that last one as written... on the other hand, I think it goes well with the earlier idea of making all immortals immune to ABILITIES below a certain tier (in order to counteract abilities that don't scale well).
Surely they mean anyone Level 20 or lower? Epic Tier is 21-30 after all.
But if a level 1 peon grabs an epic sword of awesomeness, I think that should count for something.
Its easy to use the higher of Artifact Level or Character Level for this type of thing.
EDIT: Ooh! I just noticed an awesome thing about Tiamat! Each head is dazed/stunned SEPARATELY. And combined with her +5 saving throws, keeping her from doing what she wants will be nigh-impossible.
Seems a no-brainer, but good that they included it.