I've been working on a concept of a Melee Combat Warlock which well eventually multi-class with Paladin. 

The obvious course is a Pact Boon for The Pact of the Blade.  I have a DM who strongly encourages HomeBrew content so i was thinking about creating a Home Brew Pact Boon.  In this case, the Patron of the Warlock will be the Celestial (Xanathar's guide to everything) The race will be Aasimar (Volvo's guide to monsters.) The Archetype will be a Protector Aasimar. 

Here are my rough thoughts. 

Pact of the Three Staffs (Pact Boon)

You are given the Staff of The Celestial as a symbol of your pact. You can use your action in combat to transmute your “Staff of The Celestial” into one of three forms.

The Scythe of the Celestial

• The Staff glows in a purple aura and a three foot ethereal blade completes the shape of a scythe, turning the staff into a slashing weapon. This weapon does 1d10 Slashing Damage + 1d4 Force Damage.

 The Maul of the Celestial

• The Staff glows in a purple aura and an eight inch diameter ethereal spiked ball stands atop completing the shape of the maul, turning the staff into a bludgeoning weapon. This weapon does 1d10 Bludgeoning Damage + 1d4 Force Damage.

 The Trident of the Celestial

• The Staff glows in a purple aura and three mighty ethereal barbs jet forward from the top completing the trident shape, turning this weapon into a piercing weapon. This weapon does 1d10 Piercing Damage + 1d4 Force Damage.

 You are proficient with all three weapons while you wield them. On any turn in combat as your action you can transmute the Staff of the Celestial from one form to another. This weapon counts as magical for the purpose of overcoming resistance and immunity to non-magical attacks and damage. Your weapon will revert to its Staff form if more than 5 feet away or touched by any other creature (accept while being attacked). In its staff form it should be treated like as if a silvered Quarterstaff. The Staff of Celestial in its staff form may act as a spell focus. If the Staff of the Celestial is lost or broken it can be reformed in the Ritual of Celestial Mending.

The Race and Class choices are more based on the RP dynamic. I would appreciate feedback on the Pact Boon... Is it too OP? Is it too Nerfed? is it to close to the Pact of the Blade? 

1d10+1d4>2d6.  I'd drop it down to 1d8+1d4 and make it a bonus action to change. 

i agree with the idea of making it a bonus action.  As far as the damage goes, I've also considered removing the force damage all together would this make it more balanced? a straight 1d10 of damage based on the form. 

Is it one-handed or two-handed?  Is it heavy?  Does it have reach? 

in its staff form i'm picturing it as little more than silvered quarterstaff perhaps with intriguer carvings it will do the standard Silvered Quarterstaff damage (1d6 for standard and 1d8 if versatile) It would also serve as Spell Focus.

Once transformed it would loose its Spell Focus quality and simply be a Melee Weapon. All three Versions would be considered Two Handed Weapons. As far as a physical description goes i picture the staff still being the main component such as the the Shaft of a Spear. The blade of the Scythe the Crown of the Trident and the Head of the Maul would be made from ethereal energy. Probably semi translucent with a minor blue or purple glow. Because the transformation is made from energy the weight of the staff would not change regardless of its configuration, because of this  I did not instead on making it Heavy.  although all of these items potentially could have Reach, none of them seem to in the default game. Tridents, and Mauls do not have reach ... and to my knowledge a Scythe is not an official weapon (i believe a sickle is a small version of scythe.) Also, although a standard Trident can be thrown, this could not be do to the fact that it will return to staff form if more then 5 feet away from the user. 

I have decided to give the default weapons a straight 1d10 of damage for there respective types (removing the force damage from each) in favor of using some HomeBrew eldritch invocations to upgrade the weapons. I noticed that all of the other Pacts have Eldritch Invocations that link or improve the ability so i thought it would help balance the pact, and allow a player to favor one form of the weapon over another.  What are your thoughts?

Eldritch Invocations

Celestial Scythe Mastery (Requires Pact of the Three Staffs)

• Taking large graceful swings becomes devastating to hordes of enemies. Instead of a single target taking 1d10 Slashing Damage, all creatures within 5 feet take 1d6 Slashing Damage. Roll a single dice and apply damage to each creature.

Celestial Maul Mastery (Require Pact of the Three Staffs)

• Taking advantage of an enemy’s weakness can be crushing. Through a powerful overhead swing, a hitting strike on a prone enemy gives an automatic critical hit.

Celestial Trident Mastery (Require Pacts of the Three Staffs)

• Through strength of mind and strength of arms … the Celestial Trident now pierces the enemy’s armor. Reduce enemies AC by your Strength Modifier during Attack roll