Fro those interested, these feats are from Magic of Incarnum.
- I'm inclined to say that you need a third feat that actually grants you essentia to use this "combo". The globe of darkness that you can create has a radius of 5 feet per essentia invested into the Shadow Mantle, and neither of the feats grants essentia.
- The globe of darkness will act as a nice beacon for area of effect spells and abilities. Sure, a rogue is strong against most of those, but if used in masses...
- The soulmeld can be dispelled like a magical item.
- If these feats are available, chances are that other material from Magic of Incarnum are available, as well, for friend or foe. There can be some enemies who will be able to battle the soulmeld's effect.
- While the Soulmeld is bound to your shoulders, you cannot benefit from magical cloaks, capes or mantles.