A few more albums I would whole heartedly recommend that haven't been mentioned yet. Some of these are relatively easy to acquire, others a bit more difficult:
Sacramentum Far Beyond the Sun- I cannot emphasize enough the excellence and originality of this album. It can easily stand with the best works of The Chasm or Necrophobic for brilliant, classically inspired death metal.
Golem Eternity: The Weeping Horizons- A unique combination of feral rhythms and wildly creative melodies, with an obscure and mechanistic sense of the epic. Listening to this evokes the image of a panzer division laying waste to all comers across a bleak and frozen landscape.
Massacra Final Holocaust- Probably the best album to emerge in the 80's, and the first band to recognize the violent, epic possibilities inherent in a reintegration of melodic sensibility into Slayer-style structural deconstruction.
Fleshcrawl Descend Into the Absurd- This is notable for the expert manipulation of tempo, dynamic and subtle melodicism to create a sense of tension and collapse within a narrative framework, which leads to a work that is both achingly beautiful and terrifyingly nihilistic...if you can hack the volume.
Morgoth The Eternal Fall/Resurrection Absurd- Two ep's from 1990 and 1989, respectively. One of the earliest bands to explore the use of doomy and melodic passages to drive death metal beyond the reductiveness of pure rhythm music. Not articulate as later expounders of the style, but the raw sense of dark passion is intriguing in and of itself.
Wicked Innocence Ominpotence- Totally different music, but from a similar headspace as Demilich. Wildly technical and possessed of a darkly psychodelic approach to both melody and harmonic progression, sort of a ringside view of a descent into madness.
Also:
Amorphis The Karelian Isthmus
At the Gates Gardens of Grief
God Macabre The Winterlong
Fester Winter of Sin
DarkThrone Soulside Journey
Disincarnate Dreams of the Carrion Kind
Ancient Rites Blasfemia Eternal