Definitely Devour (Attunement of Hunger) with Nezarec's Sin. Maybe a Gnawing Hunger with Tap the Trigger or other stability perks, a Bad Omens, whatever Special you'd like. I'd roll my armor for high Discipline, use Oppressive Darkness, and whatever other mods you prefer. Other than that, that's about all I know of Warlocks.

My personal favourite at the minute is Well Of Radiance with Nezerec's, Gnawing Hunger and Falling Guillotine. The void kills refresh everything really quickly. Alternatively, I like running Chaos Reach with Getaway Artist. With this build you can get a Sentient Arc Soul at all times with a good Discepline stat as well as Arc Traces, and it absolutely destroys red bars. It is a lot of fun in Grenadier, Arc singe Strike playlists. Both require exotics, so not hugely helpful for a new player. But, you can still run Sword + Well Of Radiance. It's the go-to for boss DPS this season.

Attunement of Chaos. The artifact gives you Oppressive Darkness and you can use that with Vortex grenades to weaken bosses. General tips: Always go high on Mobility, Recovery and Discipline. Resilience does next to nothing. Burst Glide is the better Glide. Nezarec’s Sin is one of the best exotics for Voidwalker.

Personally, I use the Devour tree for Nova Bomb (heals you to full after every kill while active). I usually pair it with a 900rpm energy SMG, a sniper/shotgun, and any heavy of your choice.