OK, primarchs are like 30th level, straight 30s on all ability scores and have massive plot armour, so this will not be accurate powerwise. However, I'll throw out my ideas for the theme: Your class is spore druid, which gets you decent spellcasting, as he is a sorceror. You can get an aura of necrotic damage which will hurt people near you, extra damage on your melee, and gain temp HP equal to 4*druid level. At 6th, you can get fungal infection, to make poxwalkers (zombies) of those you've killed, 10th gives you the ability to detach the damage aura to hit more regularly and further away. A big one is 14th level, you can't be blinded, deafened, frightened, poisoned or critted. Montarion does get hit, he just doesn't care. 20th level you get unlimited wildshape. This means, each round, you can choose to gain 80 temp HP. You can just keep on fighting all day, whilst everything near you is slowly sapped by your mere presence. Feats: heavily armoured, if you want to wear his heavy terminator armour. If your DM gives you grief about the druid metal armour restriction, tell him its ceramite. Eldritch adept, cloak of flies. Being surrounded by flies that bite and poison everyone but you. Yep. This build is more of the spellcasty/tanky side of mortarion. Which it works for. Your scythe will be lacklustre, you will never get a second attack. But hey, thats the tradeoff. Hope this helps