Khaim said:He is listed as level 26, which would just replace any instances of HD.
Hmm...
350 - 26 * 8 Con = 350 - 208 = 142 = 5 * 26 + 12
So if we assume Con adds to HP the same way, it looks like he has 5 hp/level plus a 12hp bonus. That, or d10 HD = 5.5 hp/level with a bit of a rounding error.
If you assume 3 hit dice at 'first level' you get:
28 x 4.5 (d4) = 126
con bonus of 8 x 28 =224
126 + 224 = 350