So i want o make an assassin character, with illusion spells for crown control. My question is, will i need to level magicka for spells like Hysteria.

Why wouldn't you? I'm playing a mage/thief and finding that magic is so effective a combo with stealth that I put way more into magicka than either health and especially stamina.

I don't have the game yet, but I don't think you need to level magicka much at all. Unless you plan on taking a while to kill your enemies then you do need to increase magicka for constant uses.