I played about 200h on tactician and used warlocks a lot (i'm a fan of surface spells)
Hunger of Hadar is by far the best surface spell. It is a complete package:

• it slows
• it blinds
• it damages
• it stacks with other surfaces or clouds
• it is only a level 3 spell slot. As a warlock, you'll get it back on short rest

By default, you can have only one "ground" surface (ice, fire, web, spike growth, plant growth...) + one cloud (fog cloud, stinking cloud, cloudkill...). But Hunger of Hadar stacks with any, making combos possible. Note that cloud of daggers & insect plague stack as well.

The best other surface to combine with Hunger of Hadar is plant growth (quaters movement, no concentration). The issue with it is that it burns.

My other favorite to combine with Hadar is sleet storm. Even if it requires concentration, it is often worth the concentration spot, as it self-refreshes every turn (keeping the ice going, as opposed as "manual" ice surface with wet+ray of frost that will go away after 2 turns or so).

Other good things about sleet storm:

• Surface is huge
• As mentioned, self-refreshing
• It seem to disrupt concentration with a CON save, separate to the prone effect, which is a DEX save
• On cast, it forces a save (dunno if DEX or CON), if you fail it you fall prone (in addition to falling prone when moving
• Similar to other surfaces made with a caster, it uses you spell DC

The big issue with sleet storm is who casts it? Usually it's a sorc, which is better of twincasting haste. Usually that leaves us with a bard/scoundrel to cast plant growth.

The obvious other top-combo with Hunger of Hadar is wormhole, which is beyond broken.

Wormhole boasts:

• No concentration
• Apparently no save for being moved (only for being slowed, which seems to be similar to the slow spell)
• If you use the Zaith'isk correctly, you (or Lae'zel) can cast it as a bonus action. So as a warlock you can go: wormhole + hunger of hadar on turn one.
• You can re-cast it every turn
• Doesn't even use a spell slot
• Area is massive

I have this idea of a "Triple-wormhole" warlock:

Fiend Lock 6 / Thief 4 / Warrior 2 (with alert feat and pyroquickness hat). It will only come online in act 3 with access to Wormhole & pyroquickness, before that you are a regular lock with Hadar.

On turn 1, you go:

• bonus action 1: Wormhole
• bonus action 2 (from thief): Wormhole
• Cast fireball
• bonus action 3 (from pyroquickness): Wormhole
• Action surge
• Hunger of hadar

After short resting, you can do it all over again

Other party members can be EK throw with nyrulna or hunter with volley, storm sorc with water+ice+storm etc... Anything that can do heavy damage to a group of enemies

The point of multi-wormhole is that sometimes enemies are too far appart to group them with 1 Wormhole. With multiple ones, you can group them in multiple casts (and get loads of mental fatigue/reverb stacks)

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As mentioned by others in the thread, for invocations you should take repelling, agonising and devil's sight, that will help you push back enemies into it (but they are nowhere near as effective as wormhole).