A black wooden egg cup, very hard, smelling a little of charred wood. It is resistant to fire (it needs to be: see magical properties) and and it is delicately carved with relief images of fruit and vines and other fertility symbols.
An ordinary egg is inserted into the egg cup of summoning. The cup is placed in a lighted hearth and the egg slowly begins to warm up. Incantations are read, and powders thrown onto the fire, and the egg will hatch into the summoned creature.
Addled eggs produce deformed demons or chaotic familiars. This can be troublesome.
Double yolks produce twin familiars, and larger eggs naturally yield larger summonees.