Kalub

Kalub was a scientist experimenting with time travel.

He created a creature and trapped it in a bubble of energy, so when it tried to escape, it would create energies that could be harvested as a perpetual source of power. His experiments eventually attracted the attention of the Time Lords, but they could not act directly because they were trying to hide from their enemies after the Last Great Time War, so they enlisted Missy because they had foreseen a potential time where his creation would escape from the containment unit and cause damage to history.

He was eventually killed by Yayani. (PROSE: Lords and Masters)