Mysticism. Bethesda got rid of it because it was OP. Consisted of teleporting, absorbing stuff (mostly health), detecting stuff, telekinesis, soul-trap... what more do you need (besides the other awesome spells also rap£d from Alteration)? In the end the absorption spells got sent to Restoration, detection and telekinesis got sent to Alteration, soul-trap got sent to Conjuration, and the teleporting was removed entirely.
Back in Morrowind I didn't even use Destruction because the only spell-schools I needed were Mysticism and Alteration.
These days Mysticism doesn't exist and Alteration has had most of the good stuff removed... and we're left only with the few good things left over from Mysticism (the defensive Alteration spells were always crap). Even the nerfed touch-only version of Absorb Health left in Oblivion has now been removed because it was actually useful for something.

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I'm quite enjoying my Illusionist. It's my second mage type character. Conjuration was alot of fun too, on my Necromancer. Both characters have used alteration/destruction/restoration, but those feel like more of utility schools to me.
The most entertaining moments with illusion come when casting the calm spell on enemy npcs. Says the group of Thalmor assassins dispatched to kill me, "Stupid spell, it didn't even do anything!" as they resume their patrol through the woods.
I take it you've never tried dual-cast Frenzy then?
... or the master spell Mayhem.
I killed a dragon in Solitude the other day.
Every fetcher in the city came to have a look at my disintegrated pile of dragon-ash.
I was bored.
I cast Mayhem.
I had to get the hell out of there, as there were over 20 people in a free-for-all bloodbath killing the hell out of each other until only the 5 or so quest-essential guys were left.... and they'd have a little rest, then start it up again. Was totally hilarious. I sat on a nearby rooftop and watched it all play out. I even saw a little kid pull out a knife and run in to join the fight! I squee'd when I saw that!
