Undead with class levels...

[Man-thing]

If I have an non-templated undead take class levels, does he get d12's or the class dice?

All templated undead that can gain levels seem to have "all current and future HD are d12" but non-templated don't clarify if they follow that pattern or not.

 

[Infiniti2000]

Does the undead you're considering say "Advancement: By character class"? If not, then they obviously didn't consider such a thing happening. Regardless, I'd use the typical undead template rule and just always assign d12 HD. Also, make sure you use the elite array (for PC classes) and non-elite array (for NPC classes).

 

[Caliban]

If it's an Undead template, check the template. Some (like the lich) convert all current and future HD to D12's.

If it's a non-template undead, it gains HD from the class. (Check the mummy entry in the MM.)

 

[Man-thing]

Thanks.

I has Advancedment by Class. But doesn't mention that sort of thing.
It's not a template so I will follow the Mummy Lord pattern