Summoning calls an entity from beyond the physical world — a spirit, an elemental force, or something worse — and compels or bargains it into service. It is two acts, not one, and the second is the hard part: bringing a thing across is a matter of the correct invocation, while holding it to terms once it has arrived is a matter of what the summoner brought to the bargain.
It is the most dangerous of the spirit disciplines for exactly that reason. What answers a summons may be hostile, capricious, or far stronger than the one who called it, and it need not announce which. A summoner who cannot bind what they have called has not made a mistake in a roll; they have introduced a free and unfriendly power into the room.
Invoking it. A summoning is a Success Test like any other Mystical Ability, and it is spirit-governed: it draws its EML from the Spirit Power it is performed through rather than from a skill of its own, so there is no summoning without a spirit whose conduit the summoner already holds. A summoned entity that is not simply obedient is dealt with as an opposed test of the summoner’s command against the entity’s own resistance.
What bears on the test. The Spirit Power the summons works through; the preparation, the place, and whatever the summoner offers or threatens; and the standing and nature of the thing called, since a greater entity is harder both to call and to hold. Nothing may be summoned while the character carries Aural Shock, and no Mystical Ability test may be fated — the bargain is struck on its own terms, and luck gets no say in whether it answers.
The form is set out under Spirit Action, and the channel it works through under Spirit Power.