"This… this is the same Goblin Lord that I fought with, right?"
When Eren got to his next victim, the goblin lord that was slumbering inside the array looked familiar to him. Then he remembered that it was the same Goblin Lord that Kat and the rest had managed to lure out of his monster nest. He had then killed the mana body of this Goblin Lord, giving everybody a hefty amount of Titus points.
Eren didn't touch the F-Rank mana beast and monsters because they were no use to him. And he had killed and drained the life essence out of a bunch of mana beasts and monsters in the E-Rank.
He soon realized that he wasn't using Life Drain and his current situation efficiently. He was subconsciously avoiding the mana beasts in the D-Rank when they were all slumbering and of no danger to him.
Eren quickly stopped using Life Drain on E-Rank monsters and mana beasts and instead used his ability on D-Rank monsters. The Goblin Lord was going to be his first victim in that segment.