The name of the spirit would be invoked, as well as some sort of protection for the spell caster against from the spirit's vengeance. Then there would be a snippet for the expected action of the spirit, as well as the target. The basic structure of a spell would be something like:
In the name of (insert name of relevant god(s) here), I call thee forth, (name of spirit), to honor the duties bound to thee by (name of Archon or other power) for time eternal. (Perform this action), (On this target) (for this duration)
A concrete example would be
In the name of Gaia, mother of stone and steel, I call thee forth Eahmat, to honor the duties bound to thee by Aesther, Archon of Earth for time eternal. I hold thee to the oath yea has taken to seek no retribution on I, Carslen of Harrow, by whom you have been summoned. Hold fast this portal, that none may pass, until I release thee from thy task!
There might be a symbol or three required, such as one for Gaia, one for Aesther, and one for Eahmat. The spell written may or may not include these symbols.
There might be a spell component that is referred to in code, such as "a pinch of the metal used by Aesther to cast the Gates of Banla"
Thus the rolled chance for understanding a spell. If the spell says "use the symbol of Aesther", and the caster doesn't know the symbol, then the spell can't be cast. (At least not safely). Likewise, if you don't know what kind of metal the Gates of Banla were made from, you are also out of luck.
It isn't important that the player or the Labyrinth Lord know the exact wording of a particular spell. But the more detail there is, the more opportunity for a caster to go in search of a particular piece of knowledge in order to cast a spell that he/she has found. Particularly if they rolled high and don't understand the spell.
Suppose for example that the caster knows everything except for the symbol for this particular spirit, Eahmat. A search for such a symbol might start at the nearest temple, asking the priest if (s)he has knowledge of such a spirit. This might lead to traveling to a particular city, town, or abbey that has a library that might hold such knowledge.
Not every quest has to be for an artifact of power that can shake the foundations of the earth.