You carefully inscribe a glyph into the floor, tying another spell to the glyph. Increase the casting time of this spell by two minutes per Power of spell being tied to the Warding Glyph.

Whenever any creature enters the square, there is a chance that they step on the glyph. The Director will secretly roll a die. If the result is an odd number, the creature has triggered the glyph. They will be affected by the spell tied to that glyph. If the companion spell would require a Spellcasting skill check against a defence, the target may make a saving throw of that defence's controlling attribute against your Spellcasting skill TN.