A spring-loaded security measure that fires a spear at whoever sets off the trap. One notably dispatched Thinfingers of the Undercity Thieves Guild.

A shame he didn’t see the trap that got him. But a bloodspear’s pretty painless if the spring is cranked back hard enough.“ - Thieves Guild guard, Backstory Codex - Lost Soul (RPA-BC) 

A bloodspear is the reported cause of death of your adopted son Thinfingers on a run with the Orc Thieves Guild guard.