Anti-matter organism

The anti-matter organism  was an intelligent blob of energy, capable of existing in both the universes of matter and anti-matter. The organism was used by Omega to hunt down the Doctor and, acting as a bridge, take him to the anti-matter universe.
 * See also the Gell guards (the other anti-matter minions of Omega) and the Anti-matter Creature of Zeta Minor.

However, the presence of more than one Time Lord confused the organism, causing it to send more than one individual into Omega's domain. (TV: The Three Doctors)