WeakAuras can probably do that with a custom trigger.
But I would say that a macro would do the job as well.
Code:
/run mds=CreateFrame("Frame")mds:SetScript("OnEvent",function(s,e,...)if select(2,...)=="UNIT_DIED" and select(9,...)=="Majordomo Staghelm" then SendChatMessage("SIPHON!","WHISPER",nil,"caster_name")end end)mds:RegisterEvent("COMBAT_LOG_EVENT_UNFILTERED")