I got the underking ending, because I wanted the necromancers amulet.Does the amulet run out of charge like any other enchanted thing, or does it keep its powers forever?
Also, can you actually MEET the underking in person?

And, are the guild quest worth doing? Do they have an interesting plot when you advance more(like oblivion morrowind), or is it just doing random quests forever and no plot? Because im a defender in the FG and theres no plot so far

Afaik there's no plot in any of the guilds but I recommend you to do some Knights of the dragon quests because you can find some artifacts there.

Im a member of knights of the rose.do they have artifacts?

also, i still want answers to my other questions

I got the underking ending, because I wanted the necromancers amulet.Does the amulet run out of charge like any other enchanted thing, or does it keep its powers forever?

Yes, it can wear down and break just like any other magical item/artifact.

Also, can you actually MEET the underking in person?

No, especially since he's now finally dead (you gave him the mantella).

And, are the guild quest worth doing? Do they have an interesting plot when you advance more(like oblivion morrowind), or is it just doing random quests forever and no plot?

Certainly they are worth doing. There is no plot to them, they are random, although some quests you only get once you are higher ranked. But really, I like that the guilds don't have a plot; they're just jobs. I'm not some super heroic dude coming to save each guilds from doom; I'm just another potential trying to climb the ranks. I'm doing the sorts of jobs I would expect a guild to give me. And it adds to the replay value.

[edit] I'll add that the guild quests are also worth doing for the perks of higher ranks.

Im a member of knights of the rose.do they have artifacts?

Yes, all knightly orders offer the same quests, including artifact quests.