Unburstable is a skill in the Spiritual Guide skill tree which automatically revives the shaman upon death. This skill only has one skill point that is able to be allocated and only works once, meaning that dying once more will not revive your shaman.
You do not respawn where you started, but instead you respawn where the mice spawned at the beginning of the map (assuming the 2 locations are different). Some shamans use this to their advantage on tricky maps by purposefully dying so they will appear next to everyone else.
Unburstable does not revive a shaman upon death in Dual Shaman maps. In these types of maps, each shaman has one life, even if they have the Unburstable skill.