Its once per round.
If 3 creatures die in a round you only get mana once.
But with this wording you dont get to choose when to use it.
So you cannot delay this effect if a low level creature dies early in the round, in case you hope to kill a high level creature with little HP.
On the paper its a nice spawnpoint, but time will tell.
I can see this one being cast in mid game instead of as an opening, and it definately has synergies with altar - it will be intersting to see if its enough to make it a viable combo.