Where the summoned creatures come from?

I cannot say if this was ever explained in either Morrowind or Oblivion, but it is later explained in the Skyrim DLC Dawnguard. The Plane of Oblivion you are looking for is Soul Cairn.

It is a plane of Oblivion inhabited by souls and undead

Background

When a soul is trapped in a Black Soul Gem and its energy used to power an enchantment, the remnant—the soul itself—is sent to the Soul Cairn. It is a place devoid of life—the only beings that exist there are souls, undead and the rulers of the realm, the Ideal Masters