No, please don't start more threads just for one question. It tends to clog up the boards, making it hard to find things. People will go to page 2 to answer a question on here, don't worry
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As for mana source ability for EGDs, it is dependant on the total damage dealt. So if you have more than one target, it will do more total damage, and gain more mana. If you have more EGD's, it will do more damage.
If you finish off a stack of one peasant though, it will do just that little bit of mana, and net minimal mana. You can often get 3 or 4 stacks with it late game, and is a great way to fuel a combat, and is the rival of the black and red special abilities. Less damage, but can often fill your mana to the brim!