@nunarita It's been a hot minute, so it's not surprising you've forgotten, but the powers people have in this world are defined by their mark. The MC has the mark of the Necromancer, which is where all his abilities come from. Apparently the Demon Lord of Greed has the ability to steal the marks of others and take them for himself. That's why the assassin girl was taken off-guard, she was trying to narrow down what mark he had so she would know what to expect. She assumed he was an assassin because he had Concealment, but the only people who should be capable of Wind Slash and other similar techniques are Martial Artists and Brawlers. Then he used spirit magic, which is an entirely separate third thing. That is, presumably, why she lost so easily. He used abilities she had no reason to believe he possessed, which took her off-guard.
I don't know if losing your mark kills you or not, the ability to steal marks is pretty unprecedented. I'm pretty sure what's actually happening is that she used the ring to send out a warning, but the ring will kill her if she's not saved, because it sends a warning by making you a ghost. I may be wrong, though, since again, we have no clue what having your mark stolen does to somebody. This is the first time it's come up.
It's actually not surprising that the Demon Lord of Greed has an ability like that. The real question is, if that's what he has, what do the other Demon Lords have? We're about to see what powers Lust possesses.
[EDIT]
I'm sorry, I sort of told you wrong. I've been rereading this, and everyone is defined by two things: their occupation and their ability. A person's occupation determines what they're good at, and their ability is defined by their mark. The assassin girl, for instance, has the occupation of assassin but her ability is Item Master, which allows her to use items far more effectively than anyone else. Abilities and occupations are handed out to everyone by the gods, even demons have them. Someone who does not have one is called a Covenant Seal Breaker, and is reviled universally. The idiot hero became a Covenant Seal Breaker when he behaved in an evil manner, because his occupation and his ability were both contingent upon his being Good aligned; when he threw his companions under the bus and disregarded their lives as unimportant, he lost both his occupation as a Hero class and his ability Blessed Oracle. This is why humanity executed him.
I assume that what the Demon Lord of Greed is doing is stealing people's Abilities. Presumably, he has the Occupation: Demon Lord of Greed, and that's probably what gives him the power to do this. The multiple abilities he has displayed are from having stolen multiple abilities from other people. However, either the translation of this series is a little wonky or the author has had some trouble keeping this straight, because the assassin girl presumed he turned invisible because he had the assassin class, but then he threw out wind cutter, which belongs to the Monk and Brawler classes. This has nothing to do with abilities, it sounds like classes. I would assume he's stealing people's classes and not their abilities, were it not for the fact that he explicitly stole her Item Master from her, and not her Assassin. Maybe he can steal both? I'm not completely sure what's going on. We may have to wait for the next chapter to get some clarification.