Redfang spawns after the last screecher bat on the ground floor is killed.
Redfang spawns four blooddrinker bats (hit for ~2000; unmezzable; unsnareable) upon being aggro'd.
Four minutes after spawning, Redfang despawns. When this happens, numerous voracious bats (hit for ~1200; flurry) appear and attack. These are killed simply by rooting them (though the root must last a full second or two).
One minute after this occurs, Redfang spawns again and must be aggro'd within one further minute, otherwise the event resets. Rinse and repeat until Redfang is killed.
A blood splattered chest spawns upon Redfang's death (loot from the event is listed there).