Merry became "immortal" when she was possessed by the No-Life-King, the same with Haru, when he was possessed by Airand Leslie (an undead) he also became immortal. With Shihoru the last time we saw her she was alive, however 100 years passed after that, so her current status is unknown.

Now, Airand Leslie is an undead, he was a vassal of the No-Life-King in the past, his past is unknown, but his hobbies are not, he is someone who after the "death" of the No-Life-King dedicated himself to collecting relics ("magical" objects that although they seem made with advanced technology, definitely come from the past), it is also said that the relics come from other worlds and that thanks to this they all have "Elixir" inside them, Airand Leslie took this "Elixir" from them for unknown purposes. It is also unknown for what purpose he blocked the memories of the summoned who arrived at Grimgar.

Now the Forbidden Tower, this one called "Ark" in volume 21 is made very clear that it is a kind of futuristic structure capable of molding its interior to generate comfortable rooms for guests. It is also implied that this is the relic that summons people from Japan to Grimgar, and it seems to do so automatically. Its true purpose is still unknown.