I want to know a thing, for example, for Spectre, the DC for the Fortitude save against the Energy Drain is equal to 15 and the save DC is Charisma based. Does it mean that to calculate the DC of the save they use the charisma of the spectre (which seems impossible with 15 in Charism) or the PC must do their save in using their bonus in charisma and not their constitution ?
I want to know a thing, for example, for Spectre, the DC for the Fortitude save against the Energy Drain is equal to 15 and the save DC is Charisma based. Does it mean that to calculate the DC of the save they use the charisma of the spectre (which seems impossible with 15 in Charism) or the PC must do their save in using their bonus in charisma and not their constitution ?
All that means is if the Spectres charisma goes up or down for some reason raise or lower the dc appropriately.
I want to know a thing, for example, for Spectre, the DC for the Fortitude save against the Energy Drain is equal to 15 and the save DC is Charisma based. Does it mean that to calculate the DC of the save they use the charisma of the spectre (which seems impossible with 15 in Charism) or the PC must do their save in using their bonus in charisma and not their constitution ?
Are you looking at the Spectre from the PFRPG Bestiary? If so, then the DC is 16 (= 10 + HD/2 + Cha modifier).
The formula for nearly all (ex)traordinary and (su)pernatural abilities are 10 + 1/2 HitDice + Ability Modifier. In the Spectre's case, Charisma Modifier. Poisons are usually Constitution based.
So 10 + 4 (half HD) + 2 Cha = 16.
This is described so in case you create a spectre with more HD, or a higher/lower Charisma, you can calculate the new DC.
I want to know a thing, for example, for Spectre, the DC for the Fortitude save against the Energy Drain is equal to 15 and the save DC is Charisma based.
"Charisma based" simply means if your Spectre's Charisma changes the DC of the ability changes.