'The last pillar is Liam.'
That was what Elder Rose told Abby and she knew what it meant.
Letto was the pillar, he needed to be killed in order to close the hell of gates and stop the poisonous miasma from spreading around, but they had not done it yet, because Iris insisted not to kill him and find another way to actually save the young man.
More so, they had not yet found the last pillar, in this case, there was no point of killing Letto.
But, if Liam was the last pillar and they knew about it, it would be a different story completely.
Abby was trembling when she learned about that, she felt so sick and vomiting a few times, she couldn't bring herself to believe that piece of information.
Elder Rose didn't want to tell her about it, because she wanted to spare her feeling, but how could it help her? Either she knew about it or not, it didn't make her feel better.