The Siren also has a larger area of influence around it, which grants enemies inside it a permanent resistance to the same element the Siren carries. This means enemies have a 90% resistance to the given element, but is now also immune to crowd control of that element.
It is not targetable by towers and requires hero or pet damage to be killed, and will travel from the door to the middle of a lane. Leaving it alone for a fair amount of time will allow it to flee back to its original gate.
Killing the Siren or letting it flee does not remove the elemental buff on enemies, however a fleeing Siren nets no loot drops.
Loot drop quality is similar to that of an end-wave reward chest.