D&D 3.5’s Monster Manual 4 introduced "Bloodhulks", three new kinds of mindless undead which are not templated from a base creature.
The Spell Compendium has the most updated version of the spell "Awaken Undead", which adds intelligence to an undead creature, "subject to the limitation that an undead cannot be more intelligent than is typical of a living creature of the same kind." It also gives it a bunch of feats and abilities, based on what it had when alive.
How does this spell interact with mindless undead that don’t have a base creature? I’m looking specifically at bloodhulks, but there are other mindless undead that this could apply to as well.