Nebulous
Legend
Yeah, totally right, missed that. So you could command 100 attack rolls at once, though only one of them could do damage to the tarrasque thanks to Magic Weapon's concentration (and there's nothing in the RAW prohibiting you from finding 100 magic bows that we've seen yet, but I don't think that's...realistic...given 5e's assumptions).
Cheaper, now. 100 3rd-level spellcasters + 1 20th - level necromancer to do 100 magic weapon attacks/round, but still seems kind of like army territory.
Here's the thing, if you have a god-like 20th level necromancer commanding a small army of 100 3rd level spellcasters...if you can coordinate that kind of assault i guess the Tarrasque should be given a run for its money, and possibly forced back into "coma state". Regardless, that kind of assault is unlikely, and it's going to kill a lot of mages really, really quick. I love the visualization though...