It strikes me as unlikely that the fiends can pour massive power into Vaarsuvius' body when they control it, or they'd have no need to make a deal with Vaarsuvius, since whoever they made a deal with would become ridiculously powerful when they chose. A random first-level commoner would do, or someone like Kubota. They could make the deal whenever they chose and have whoever they'd chosen Epic Teleport to the Gate right now.
: What? The snail from that generic dungeon? Here?
:heavily templated snail:: OBLITERATION.