What spell level would you say this spell should be?
I initially thought 3rd by comparing it to blur, which only affects the caster, whereas this gives disadvantage to multiple creatures no matter who they attack.
On the other hand, it's more or less like a 'reverse' faerie fire, which is only 1st level.
On the third hand (!), I've currently written that a creature may repeat the save to end the effect (which you can't do against faerie fire - a spell that I do think is pretty powerful for 1st level anyway).
(Constructive criticism welcome - I just thought I would try and expand the repertoire of illusion spells).
CASTING TIME 1 Action
RANGE/AREA 120 ft (30 ft )
COMPONENTS S, M (a silken veil)
DURATION Concentration 1 Minute
ATTACK/SAVE WIS Save
You create a pattern of shifting shadows within a 30-foot cube within range. All creatures within the area when you cast the spell must make a Wisdom saving throw. On a failed save a creature suffers disadvantage on all attack rolls as the shadows flit about their heads in a distracting manner. A creature may repeat the save at the end of each of its turns, ending the effect on a success.
Any creature within the area has advantage on Stealth checks.