Hi,
The Crimson Bat, aka the Steed of the Red Goddess is a horrific manifestation, Blaskarth, the so called Cosmic Death, a potent being of entropy. Huvaran the Bat of Death, the so called Traitor of Birds is the Rinliddi folk god of death, who in turn is associated with Blaskarth. However in the second age teh Crimson Bat does not appear to be manifested, this only occurs when the Red Goddess in her form of Teelo Estara defeated and devoured the God of Death (Blaskarth) and created the Crimson Bat from her own body. Blaskarth's magic includes communion with the Otherworld, Mastery over Bats and Emulation of Bats. This cult or more precisely the cult of Huvaran is almost certainly in existence in the Second Age.
I was always of the belief that the Crimson Bat could be slain, but that its soul would return to the Bat Cave on the Red Moon and a new body would be formed, he is after all an aspect of the Cyclic Red Moon and descendant of a Death God. So I would guage the level of the defeat your players did and then give them either seven days, seven weeks, seven seasons or seven years before the bat comes knocking at their door, and hell he may even bring his Daddy.
For more information on the Crimson Bat see ILH2:Under the Red Moon from Moon Design, or if you can find it Cults of Terror. The first is still in print and is available from Rick Meints, see www.glorantha.com.
Cheers Simon