Just a couple of little nitpicks, the regeneration at 20th level doesn't state what overcomes it so it is better than the Tarrasque's at that point, and the fast healing on the whole seems a bit excessive, I mean 15 at level 18 all day? Even 2 at level 3 is infinite self healing, which is not that bad but still could use some restrictions. Also with any necromancer you want to go undead, so what happens to the regeneration then, and hell what happens to the strain, does it just not work or function differently or does it function exactly like normal.