Stealth Spell- Cloak your troops into the darkness!

So, I haven't done one of these in a while. Let's go!
What this spell does is it, similar to the AQ's Royal Cloak, cloaks your troops into the dead of night!
I don't know what else to say, so let's go to the stats!

Radius: 5 or 6 tiles (honestly can't decide)
Housing Space: 1
Time To Create: 20 minutes
Targets: Ground and Air
Dark Spell Factory Level Required: 4
Level 1-4 stats:
Spell Duration: 8/10/12/15 seconds
Cost: 75/80/85/90
Research Cost:N/A- 50,000/ 70,000/ 95,000
Research Time: N/A- 9/12/14 days
Laboratory Level Required: N/A 7/8/8

Ok, tell me if I missed anything or what I should change!

Way too OP. Reduce the area of effect to 3.5 tiles and decrease duration by about 60% at each level, and you've got yourself a pretty nice spell.