The slow to SS might be nice while the ult's up, have it be large, like wither for the on enemy cast, or low like Command: Dissonance/Shard of True Ice on ally cast.
Ult Revision 1: I like it, a range of 200-300, a fear which calls to mind the powers of chaos but isn't OP, and threat generation through constant aoe dmg, which meshes well w/ her short range kit. Biggest issue: she's not inherently tanky, which means she could easily become the enemy team's highest priority target and die here. Maybe keep a version of the stat boost so that her max health increases for a short duration. Biggest Attribute (imo): introduces the possibility of a cool scary transformation graphic showing the power of the Void.
Ult Revision 2: Stealth for 5 seconds not as good for champ design/lore, but better for an assassin (jungler/top lane) type champ, on hit magic damage for a brief duration is an interesting idea, particularly if her q is an auto attack reset, might be hard to balance but I like the idea; I think she should either get the cc buff (slow) added to spectral swords, or an MS buff, not both (an AS buff might be more appropriate, but again balance is going to be difficult because it's a novel ability), the .5 second stun is reasonable, make it like naut passive so that it can maybe recur over the course of the ult on the same target, but so that there is adequate time between the proc so that the ability isn't OP. I would suggest either the on hit stun, or the aoe silence (attacking ends the stealth), not both.
Basically if you go option 1 she's AP nasus with really nice abilities so that she's actually useful for a team, and in roles other than top. If you go option 2 she's a bit more unique (a plus), doesn't fit her background as well (a minus), and will be hard to balance (meh), and fills some roles better (i.e. jungling) than she would w/ option 1.
Personally I would probably prefer option 1 because I like AP bruisers more than assassins. As a player I would also tend to choos option 1 because I don't glass cannons top lane or jungle, and I don't think option 2 ult would be good mid lane. As a designer I like option 2 because it's cool.