One of my player’s PC is a Ruinguard and has become an intelligent undead. Looking at the rules, it says he is now a 1 HD undead and gains XP at a rate of 4,000 xp for 2 HD, doubling each time, and as his HD increases he gains his class abilities too.
Is that right?
It seems that the new HD/XP lets the PC gain their new class features quicker than normal levelling.
I think that example is specifically speaking about a 1st level Ruinguard transformed into intelligent undead - presumably your player’s Ruinguard would be transformed into an undead of his current HD, and therefore their XP to next level would be adjusted accordingly.
I’m not sure you can downgrade a creature’s HD through necromancy.
Additionally, nothing will stop the undead ruinguard from advancing past HD 14, just doubling xp all the way. In my game I’ve been toying with capping the HD of beast henches and my ruinguard if he decides to go undead for the sake of a restoring a gameable xp curve. In exchange for capping after 8th level I’d start adding a flat amount like other player classes. Ruinguard already has the highest flat amount of any class, last timed I looked at 175k per level; I would likely increase this to 200k-225k to represent a slowing of the curve as a price for the undeath.