They summon a demon at the witches' gathering.

Literal

Witch-gathering [at-で] demon [object-を] summon.

魔女集会 ('witches' gathering/sabbath') — 魔女 ('witch') + 集会 ('gathering'). 悪魔 ('demon/devil') is 悪 ('evil') + 魔 ('demon'). 召喚する ('to summon') is a formal verb used in both legal and fantasy contexts.