A couple of comments on the heroes and monsters shown. I was provoked by the comments from Voadam, so it's all his fault.
I thought I would look at Autolycus, Jason, Sigurd, and the three creatures and look at their hit points. Some are more confusing than others, to say the least!
Autolycus is pretty average. He gets 10d6+40 for being a Rogue, and 10d8+40 for the Shadowdancer. With an average of 3.5 hp per d6 and 4.5 hp per d8, that comes to exactly 160 hp, which is what he has.
Jason didn't fair so good. He gets 10d10+30 for being a fighter, and 5d10+15 for being a Ranger. Averaging 5.5 hp per d10, that should be 127.5, or 128 hp. Jason gets 111 hp. Removing the 45 hp he gets for his CON, that means he averages 4.4 hp per d10. Guess he's not as lucky as we thought!
Sigurd is the healthy one of this lot. He gets 16d10+48 for being a Fighter. With 5.5 hp per d10, that averages to 136 hp. Sigurd gets a very healthy 175 hp! Again, subtracting the 48 hp he gets for his CON, that means he averages about 7.9 hp per d10. Very nice indeed!
Now to the critters.
Cerberus (you have it misspelled in the heading) is shown as having 10d12+35 hp. With an average of 6.5 hp per d12, that should be 100 hp average. He gets a respectable 115 hp. Subtracting the 35 hp, that means he averages 8 hp per d12. Not bad, except I think it's wrong on two accounts. First, as Voadam pointed out, an outsider should use d8 for hp, not d12. Secondly, with a CON of 17 he should get +3 hp per hit die. That would give him +30, not +35. So, using the proper formula of 10d8+30, and using the average of 4.5 hp per d8, Cerberus should have an average hp total of 75. Maxed out, he could have 110. Still a bit shy of the 115 hp shown.
Grendel is shown as having 16d12+95 hp. With an average of 6.5 hp per d12, that should be a 199 hp average. He is shown as having the quite huge total of 259 hp! Subtracting the 95 hp, that means he averages a very healthy 10.25 hp per d12! That's impressive, but I think it is wrong. Again, as Voadam pointed out, a Magical Beast should have d10 dit dice, not d12. Also, with a CON of 24 he should get +7 hp per hit die. At 16 hit dice that should be +112 hp. Using the proper formula of 16d10+112, and an average 5.5 hp per d10, Grendel should have an average hp total of 200 hp. Maxed out, he could have 272 hp.
Finally, we have the Nidhogg. Using the 40d20+340 formula as shown, and averaging 10.5 hp per d20, the average works out to be 760 hp. The Nidhogg is shown with a respectable 998 hp. That averages to 16.45 hp per d20! Now, I figured you took the d12 hit die for a dragon and bumped it up a die type to d20 because you added an outsider template to it. So, I have no problem with the 40d20. The CON of 27 should give it +8 hp per hit die, for a total of +320 hp, not +340. Given that, the Nidhogg should average 740 hp.
That's enough of me being a math geek for one day.