Primordial Spectre is a type of undead created using the reaction Accursed Spectre, exclusive to The Hexblade warlocks. This kind of spectre is created when killing a Large or Huge creature.
Primordial Spectres use Strength for their attack rolls and Charisma for their spell save DC. Note that this is the Charisma of the spectre itself, not the Charisma of the Hexblade which summoned it. This gives a DC of 11 for Devour Soul and Seize Soul and an attack roll modifier of +6 for Right Behind You and Reap Soul.
On Tactician difficulty and above, the DC is increased to 13 and the and the attack roll modifier to +8.