Upon slaying this NPC and other similar undead froglok in Old Sebilis it appears to be a +50 bonus to Frogloks of Guk ( help for the Crown of Deceit quest, which requires "Warmly" as a minimum to talk to 'an old froglok in Upper Guk.) It's much much faster than dealing with Lower Guk which only give +2 bonus to Frogloks of Guk faction increase.
As far as anyone can tell these guys dont drop anything. This is the ghoul version (around lvl 50 sk mob). The live version is the one that can drop a runebranded I believe. The picture shows the person fighting the ghoul holding a lamentation, not the ghoul.