This is such a beautiful song!
The basic chords are:
|Cm* (Ab)|Eb* |Cm* Gm|Ab (-maj7) |
*with e-flat on top
You should be able to figure the rest out for yourself - it's quite simple.
The riff goes something like this:
Bb - G - F - G Bb - G - F - G Eb - F - F - F
Source: https://tabs.ultimate-guitar.com/tab/efterklang/sedna_chords_1206393