I heard when the ice wall breaks with invocation active the broken ice wall summons evil spirits, is this true? thank you.
As a side question I wanted to know if its worth investing more than 1 point into invocation as it doesn’t seem to change at all at higher lvls except longer duration.
“If the Invocation caster destroys his icewall, it will summon the spirits. If the ice wall expires, nothing happens.”
Source:
Higher lvl invocation increase hit count and dmg dealt. At lv5, u can learn the attribute for invocation which increase all dark atk dmg by 30% which atm doesn’t work because this is Tree of Bug. 
Invocation is extremely powerful, and the damage/hit count scale with level (the scaling is same as DT except it has no attribute for damage).