The Summoner

A pact made between you and your patron grants you the ability to call fourth minions from the outer realms. A power undoubtedly given to you to help further your patrons own goals, be they benevolent or pure evil. The power can often come at a cost, but a cost you are prepared to make. Your patron can be any powerful outsider be it fiend, fey, ancient, or divine. 

Summoning

Your patron lets you expend spell slots to summon fourth a creature of your patrons domain. The creature is CR equal to the spell slot expended  and is summoned within 30 feet of the summoner. once summoned the summoner can use a bonus action to move and command the creature.

The creature can be commanded to attack, hold, use any of its abilities or to return to its home plane.

The creature remains for 1d4 plus the warlocks level rounds or it falls to 0 hp then it returns to its home plane.

Bend to my will

Starting at 1st level, when one of your summons is to return to its home plane at the end of its duration you may make a wisdom check DC 10 + CR of the creature.

A success increases the duration of the summon by a number of rounds equal to your Charisma bonus.

A failure results in the creature breaking free of your command. it remains on the current plane under it's own control.

Soul Recovery

Starting at 6th level, you may use a bonus action to dismiss a summoned creature you expended a spell slot to summon. You recover any spell slot that was used for the summoning.

You may recover a number of spell slots in this way, equal to your warlock level per long rest.

Summoning mastery

Starting at 10th level, you can choose to attempt to summon an additional creature, or a creature with a CR higher then the spell slot used. Treat the second summon or higher CR summon as though its duration is 0 and perform a "Bend to my will" check. The appropriate results apply.

 The summoner may maintain concentration on multiple summons, however any check required to maintain concentration must be made for each summon.

Planar anchor

Starting at 14th level, once per long rest when you summon a creature you may choose to expend twice the required spell slots to make the duration of the summoning permanent.
The creature remains on the plane under the command of the summoner until it or the summoner's HP is reduced to 0.