The Great Mystery of the Silver School: Gate
In the apex spell of Silver magic, the dual functions of summoning and travel become one; a two-way rift in the universe permits the caster to step through to another plane of existence, or to call any denizen of that plane to appear. Unlike the lower level spell of planar travel, many more planes than the ones enumerated in theology can be visited. The perils of both summoning and travel, as always with Silver magic, should be quite clear. The claim that the Silver Gate can also be used for time travel is less clear, but the words of the sage Dzemaclua may help illuminate this arcanum: "For know that there are planes in which the poison dagger missed King Atroban, or the comet collided with the mountains instead of the great city of Naun. And so there also must be planes in which everything happened as in our world, but a hundred years later."
The Great Mystery of the Gray School: Wish
This is it; this is The Spell. The culmination of all magic, but also its essence: your will changes the world. Unlike the world-altering powers given by stubborn genies or magical artifacts, the Wish spell is said by nearly all authorities to be foolproof. You are not convincing something to do your will in a language easily twisted, rather, it is the language of your mind that expresses your will, and so mote it be. Curious, then, that the tales of great archmages bandying about Wishes simply don't exist. On reaching the Great Gray Mystery, it seems one either Transcends or simply vanishes. The conjectures on this point could fill a book, but the most common are that: a part of the magic of the Wish obscures its own existence, shaping reality so completely that its effects seem wholly natural; the user of the Wish splits off into a childish universe of instant fulfillment, in which all others are mere puppets, while life in reality goes on; the Wish meets all your desires at once, spoken and unspoken, conscious and unconscious, and anyone with even a hint of subconscious death instinct ceases to exist. Indeed, perhaps the way to Transcend from the Wish is simply not to use it.
And that's a wrap. Oh wait ... "clerical" and "druid" magic? OK, OK, fine, but after a short break from the spell lists.