flamepaladin

Limited servants are servants that are only available for you to summon during a limited amount of time. Since it's a gacha game, things like this is quite common. And well, to be honest, even if he's not a real servant, as long as he's popular, he'll get into the game. That's just how gacha game works . In FGO, there are already a few Pseudo-servants you can summon. I think right now, Muramasa is the only pseudo-servant not yet available.

Technically, we can't summon Muramasa since he's not released yet. He has only appeared in the story. Well, looking at his popularity, he'll be released as a limited servant sooner or later.