A spell or curse put appon someone to makesure they fulfil their task and dont betray their master. For instance a Geas could be put upon a servant to make sure that they stay loyal or upon a mercenary to make sure they dont double cross their employee.
Faliure to do so will result in the victim being killed in one way or another by the Geas.
Used alot by Celtic Shamans and Norse Guayla (witch doctors). Not used offensively as any magic user able to preform a Geas can easily destroy enemies in a multitude of other ways.
'The Shaman forced my undying loyalty by putting a Geas on my soul'