In my opinion it has a lot of potential as a support/healer comparing to pure healer but I haven't got too far with DoK, so maybe someone with more experience would correct me on that. At the moment I play dual wield healing my party members when they need it and debuffing/harassing my enemies when heal is not needed that much.
I was thinking if it would be better to dump all points in PoS and then buf 3 points into Dark Rites to buff the Restore Essence a little bit but I guess it has more sense to take skills/tactics from Sacrifice.
I started trying out different skills and first I went with is Horrifying Offering tactic. I started using it and it is unbelievable (now keep in mind I'm in T2 at the moment

I will most definitely take Devour Essence. That would leave me with 1 point free if I'm not mistaken. I was thinking about warding strike but tbh you can already ignore all of the parry attacking from behind (that's how it works, right?) so that 10% doesn't seem like a good deal to spend a point on.
At this point I might try to play with different tactics. Gift of Khaine to spam on healers (60 AP on 3s internal cd) or additional heal from Transfer Essence thanks to Empowered Transfer. I still don't know what to take from the core tactics but I was thinking about Khaine's Imbuement or Restorative Burst.
I don't think this has a lot of damage potential but is strong 1v2 at the moment, mostly due to sustain than burst damage. I am trying to focus more on supporting and offhealing rather than killing people but in scenarios for example both my dmg and healing are way above average, I am sure I can get those numbers higher once I take a proper grip on it and get better items.
As for Convenants I switch between them 3 depending on the situation. My renown points are spent on toughness.