You have made your pack with a Celestial, one the great powers of Good who are committed to combatting and thwarting evil in all it’s forms. This being is most likely one of the members of the Angelic Host of Heaven, there are a variety of other powerful beings with similar motives who are dwell in the higher realms but who are not angelic in nature. Beings of this sort include the Nin Lammasu and Bel Shedu; the Lord Kulkukan; Marduk, the Binder of Winds; various of the Great Elder Dragons, and a handful of the most powerful of Council of The Elect.
Expanded Spell List
The Celestial allows you to choose from an expanded spell list when you learn Warlock spells. The following spells are added to the Warlock spell list for you.
1st Level: Divine Favor, Thunderwave
2nd Level: Calm Emotions, Lesser Restoration
3rd Level: Beacon of Hope, Spirit Guardians
4th Level: Death Ward, Guardian of Faith
5th Level: Dispel Good and Evil, Hallow
As 1st level your patron grants you the ability of a soothing, healing touch by virtue of your presence and attention. This ability acts without limit, healing those you wish once, for an amount equal to one of their own Hit Dice (though this does not act against their own use of Hit Dice). A character cannot benefit from this again until they have undergone a short or long rest.
Armor of Faith
Starting at 6th level the character may add their Wisdom modifier to their AC as a Response to an attack by an evil creature. They may also use the same bonus to any single saving throw against an spell or spell-like attack from an evil creature as well. Use of either ability means that neither can be used until after a short or a long rest.
Mantle of Heaven
At 10th Level the character has Resistance to Necrotic damage and has Advantage to all saving throws against Fear or Possession.
Upon reaching 14th Level the character can touch a target and immobilize a target until the end of the characters next turn as it is racked by a weighing and judgment of all it’s evil, selfish, and impure thoughts. If the target is not a Celestial it takes 5d10 radiant damage and 5d10 psychic damage. This feature cannot be used again until the character finishes a long rest.