Hi there, fellow metalheads
I have quite a inquiry for you now, can you help me find the best or at least iconic korean metal bands?
The more extreme the better indeed
Thanks in advance.

Sad Legend is perhaps the most obvious one: http://www.youtube.com/watch?v=ivEFoYuqnfE
Melodic black metal.

+1 Sad Legend, excellent stuff. And though I'm personally not such a big fan, Oathean is another big (relative sense, of course) extreme band in Korea.

Thanks guys, with these two i already got something pretty good, but keep the bands coming, they are appreciated

I only know of two Korean bands, so I have no idea what are the best bands. Here is a Korean band that I know of

Seed
http://www.youtube.com/watch?v=r_zCxqRRkIg

Korea has got plentiful of melodic death and black metal bands.

Oathen and Dark Mirror ov Tragedy are hugely recognised

Pyha and Holymarsh are also fairly well known bands from Korea.

As for some lesser known bands, The Crescents aren't particularly well known, but certainly worth checking out. Sunoi and Gaia are quite good too, despite the rough production quality.

Seconding Sunoi, quality music. Recommending the little known Tokkaebi (도깨비, which means troll/goblin in Korean). They are somewhat like Cannibal Corpse and have a pretty awesome album called Astronomical Phenomena.

http://www.metal-archives.com/bands/%EB%8F%84%EA%B9%A8%EB%B9%84/99947

And here's how they sound: http://www.youtube.com/watch?v=jxJiEmK8oew

A quick MA search tells us there have only ever been 180 bands out of South Korea. I don't know even think iconic could really be used to describe any of them, seeing as how they don't have a real scene.

But I know a good little thrash gem, by a band called Crash. Their debut album slays and according to one of the reviews, they are the first and most successful thrash metal band out from there. So that's something, I guess.