200khp (before he'll transform into Vesthons Draconic Abomination)
Depops if not killed in 3 minutes
Transforms into Vesthons Draconic Abomination when his health drops to around 15%.
Given Vesthon has 200khp, and has 3 minutes before he despawns, you will need a combined DPS of over 1111 to win the event.
Upon turning in the four bone pieces to Hsagra's Shade, three a lumbering zombie and Vesthon Marijakin spawn together, in front and slightly to Hsagra's Shade's left.