Also, I’ve done the math, tell me if i’m wrong but :
Invoc and Dark theurge 15 skill give 426% skill factor * 1,44 it give 613 skill factor for both skill and 31 skill point invested
If you take Ghastly trail 15 skill, and invoc 3 it give 2 (100% improve) * 310 skill factor = 620 For both skill, so a lot cheaper.
But that mean that you have to use Ghastly trail each time before launching your spirit (otherwise it will be useless). Also, for the same skill factor you need dark theurge at lvl 10, still cheaper skill wise with “only” 28 skill point (you win 4!)
Also :
Invoc or dark theurge skill can be improved max to 426% skill factor * 1,6 for enhance attribute * 1,125 for arts enhance attribute * 1,44 if you get ghastly trail = 1104,192 skill factor per spirit.
Versus for invoc level 3 : 310 * 1,6 * 1.125 * 2 = 1116. (and same for dark theurge) But you still need to invest the same amount of attributes.
Also, for those who wonder :
invocation lvl 13, dark theurge level 14 -> 405 skill factor * 2 * 1.6 * 1.125 = 1458 skill factor per spirit = 1458 * 20 (remember dark theurge double the count with an attribute) = 29.160 skill factor vs 5490 * 5 (standard AOE attack ratio) * 1.6 * 1,125 = 49410 of mastema (not counting phantom pain) but with a larger aoe and damage more divided for spirit. for only 10 skill points vs 42
So, not worth it I guess.