The primordial

the primordial makes a pact with an elder elemental, which gives the warlock power. The elder elementals aren't particularly smart, but have immense power. The warlock is given the power to make small portals to the respective elemental plane, which allows the elder elemental to influence the material plane, with an action or multiple at high levels. you also gain a few minor benefits for making the pact

Expanded Spell List

The Fiend lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Primordial Expanded Spells

Spell Level	Spells
1st	absorb elements, earth tremor
2nd	flaming sphere, scorching ray
3rd	tidal wave, meld into stone
4th	conjure minor elementals, elemental bane
5th	conjure elemental, control winds

Elder elemental's Blessing

Starting at 1st level, You are immune to being poisoned and poison damage, and as an reaction, you can become resistant to bludgeoning, piercing and slashing damage from nonmagical attacks for the next attack when you are attacked (you must use this feature before you are hit, after it's been declared that you will be attacked).

Elemental plane portal

Starting at 6th level, you can call on your patron to attack your foes through a portal you can create.

As an action, you can create a portal to the origin plane of your patron. the portal can only be used by your patron. the portal acts on initiative count 20 (losing initiative ties) and uses one of the patron's non-multiattack actions. you can use this feature once per short or long rest. the damage your patron does is reduced to 1/4 initially, your patron deals 1/2 damage at level 11, and full damage at level 16

Elemental form

Starting at 10th level, you can use an action to convert yourself into the element of your patron. this has different effects depending on which patron you chose. another action can undo the effects

leviathan:

you can enter a hostile creature's space and stop there. you can move through a space as narrow as 1 inch wide whithout squeezing. you gain a swimming speed of 120 ft

phoenix:

you can move through a space as narrow as 1 inch wide without squeezing. any creature that touches you or hits you with a melee attack while within 5 feet of you takes 2 (1d4) fire damage. you also shed bright light in a 60-foot radius and dim light for an additional 30 feet.

Elder tempest:

you can enter a hostile creature's space and stop there. you can move through a space as narrow as 1 inch wide whithout squeezing. you gain a flying speed of 60 ft

Zaratan:

you have an AC of 25, but your speed is reduced to 10, and you lose any other speeds.

elemental nature

Starting at 14th level, you can surround yourself with the primordial powers of your patron

as an action, you can empower yourself with elemental powers:
you gain +5 ac, + 60 ft on all of your movement speeds and you double the damage you deal until you cast a spell or attack. (the spell or attack gains the double damage) 

Once you use this feature, you can’t use it again until you finish a long rest.