Death/Doom?

The Chasm's earlier stuff is pretty doomy
Daylight Dies

Mar De Grises
Ras Algethi
(both are very progressive)

Swallow The Sun
 
I used to listen to quite a bit of death/doom.

Sempiternal Deathreign (as mentionned previously)
Mystic Charm
Ceremonium
Argentum (only heard Ad Interitum Funebrarum but it's pretty great)
Arcane Sun
Crimson Relic
Sorrow
Winter
Castle
Delirium
Eternal Darkness
Thorr's Hammer

I'm probably forgetting some but I think that's a good start.