Fort saves and Advanced creatures

[DSC-EricPrice]

If you advance a creature per the rules in the MM and it moves from being Large to being Huge, does the Fortitude save to resist the effects of certain attacks (such as the Fort save (DC19) against a Marilith's constrict attack) increase as well? Are there rules for this? I cant find any anywhere yet it seems reasonable that the constricting attack of a huge snake would be more difficult to resist than that of a large snake.

 

[da chicken]

The general rule for saving throw DCs for monster abilities is DC 10 + 1/2 HD + Ability mod (DMG p94). The Marilith's ability seems to be Str based (9 HD and 21 Str: 10 + 9/2 + 5 = 19) so you should increase the DC based on the new form's HD and Str.