There are quite a few different types of spellcasters in popular media, but one generally stands out even over a Necromancer. While D&D should be credited for what we normally think of when someone says "Warlock" in a game, WoW has also had a very profound effect on what most people think of.
I pulled mostly from the D&D form of Warlock, as Sarcom formed "pacts" with various powerful entities to gain his skills, but I also took a little inspiration from WoW and put a different twist on the bits I pulled from that.
To start, Sarcom has a different way of utilizing mana than the other spellcasters. He has a higher maximum mana pool and all of his spells cost a percentage of his total mana to cast. This makes it so that you can't just throw on a mana regen accessory and be set like the others but he has a skill to take care of that issue while not losing too much damage from taking the turn to do it. I'll go through his skills and you'll get an idea of how he plays.
Life Siphon - Costs 8% mana, 10 turn cooldown. Applies the "Link" effect to a target for 10 turns. While linked, each turn the target loses 5% of their maximum HP per turn while Sarcom gains the same amount.
Essence Tap - Costs 0 mana, 4 turn cooldown. Damage a target and gain 10% of the target's total health as mana. (The attack doesn't deal 10% of the target's health, that's just what the game checks in the formula to add mana for Sarcom)
Ancient Spores - Costs 11% mana, 5 turn cooldown. Apply poison to all foes.
Dark Waters - Costs 15% mana, 2 turn cooldown. Damages the foe. Has a 50% chance to apply the "Harsh Chill" status effect, and a 35% chance to apply the "Frozen" status effect.
Ward of Vas - Costs 10% mana, 6 turn cooldown. Applies the "Ward" effect to Sarcom for 3 turns. If struck by an attack, Sarcom will reflect the damage back at the foe.
Doom - Costs 17% mana, 50 turn cooldown. Deals damage and applies the "Doom" status effect to the foe. After 15 turns the foe dies. (This doesn't work on enemies like the Challenge bosses, for obvious reasons, but it does work on almost everything else to cap the number of turns a battle can drag on for if you're really struggling with any of them)
Chaos Flare - Costs 19% mana, 0 cooldown. Deals damage to all foes. It's chance to crit starts at 50% and increases with the health of the enemy up to a guaranteed crit.
Madness - Costs 35% mana, 10 turn cooldown. Damage the target foe. Applies the "Madness" effect to it for 4 to 6 turns. While affected the target with either not attack (30% chance), attack itself (40% chance), or perform it's attack normally. (Like Doom, this doesn't work on Challenge bosses)
As you can tell he burns through mana pretty quickly if you start spamming Chaos Flare. On most bosses you can get away with that as 10% of 100,000+ health is going to max out your mana bar, but on normal enemies it's going to take a bit of restraint so you're not going "OOM" every other fodder enemy.
Unlike other story companions Sarcom doesn't have a companion quest. With the way I wrote the story for this batch of content he's already completed that and got the items. The entire main story for this content is a followup to deal with the consequences of what he did to complete that quest he set out on.