The weird part is that it's part of the aftermath. Actually, that may only be the damage multiplier for the skillchain itself, but this says it will increase burst damage too... seems to me avatar would benefit from that.
Edit: Ok had to find the exacts:
Quote:
A two-stage skillchain (created by two Weapon Skills) will result in a 30% increase in magic damage on the magic burst.
All stages beyond that will result in an additional 5% damage increase on magic bursted spells (a three-stage skillchain will yield a 35% damage increase, four-stage will yield a 40% damage increase, etc.).
So length of chain rather than tier affects MB. However, since this says it increases magic burst potency... not sure if it's some aftermath buff or part of the skillchain itself. I'd lean toward skillchain itself
in which case avatars magic bursts would be affected.