"Each time a different enemy creature is attacked and damaged by a friendly creature, place 1 mana on Pentagram. Pentagram may gain a maximum of 2 mana this way per round. During the deployment phase, if there is 2 or more mana on Pentagram, it may summon 1 dark living creature."
Read literally, once a it triggers for one creature, it cannot ever trigger for that creature again, since that creature is no longer 'different', it is the same as a creature that has been targeted by Pentagram before. The intent could be made explicit by placing the phrase "this round" or "this game" at the end of the first sentence, but I don't know which phrase I should use.