So my player wants to play a hexblade warlock. So I need to come up with his patron. The idea I had was a sentient weapon that has lost it's power over time. Now it is a mundane silver weapon, but as the player plays and grows and develops the weapon awakens and grows unlocking power over time. I don't want to tie it to levels, but roleplaying moments. My main question is how should I determine the power levels of the weapon as it grows?

I like this idea, one thought that popped into my head immediately was to set goals for the weapons, and have each goal it meets level it up to the next category, but also have each goal get progressively harder. 

For instance if the weapon was NE aligned for instance and it was trying to corrupt the Lawful character over time it might keep trying to make a case for petty theft. Once it persuaded the character to commit the theft it would grow from a normal weapon to an uncommon magic item.  This could happen somewhere between 5th and 10th level for the character and you could have it be the equivalent of an uncommon weapon, +1 weapon, or maybe a daily ability that gives the player an extra effect to play with. 

Then the weapon would move on to it's next phase of corruption, maybe tricking the player into assisting an evil organization or doing something that will lead to a longer term effect throughout the lands. Once it completes this goal it goes up again from uncommon to rare, this would happen sometime between 11th and 16th level for the character ideally. Now maybe you tack on that +1 if you added a daily effect before, or you give the weapon a short rest ability or charges to cast a spell like ability. 

I think I have made the point, ultimately the sword reaches Very rare levels between character level 17th and 20th and adds on some basic permanent abilities, fire resistance, every time on a crit it slices off a limb, that sorta thing. 

That sounds cool thanks. Yeah the biggest problem has been coming up with the abilities.

Or, you could have it be fully useful and get the whole party into so much trouble that they might not want to keep it.  Read The Oathbound, Oathbreakers, and Oathblood by Mercedes Lackey. 

You could also model it after the rules for ancestral weapons. Just make the weapon start out at the "No Generations" tier, and every time you want to unlock more of the sword's power, just bump it to the next tier. Basically like this:

• First Form: Standard Silvered Weapon that maybe happens to talk or something.
• Second Form: Now grants a feat and counts as a  magical weapon.
• Third Form: The sword now has two charges that recharge at dawn and can spend them to cast a few spells rolled from the table I linked to.
• Final Form: Sword becomes a +1 magical weapon, gets another charge, and one more spell from the stronger spell table. Also grants one of four other thematic abilities.

This way, you start small and grow over time just like you wanted.

I have gone with something a little different as the aim is to make a sentiant weapon with enough power to be a patron to a warlock. I decided to use as a basis one of the old Vestiges from the 3.5 Binder Class. Hence why some abilites may be reminicent to those who played in that edition. Also I have added more abilties than those in the templates people recoomended but that is due to the fact that for a number of the tiers (2 & 3) the abilities were not that powerful (tier 2 is an old 3.5 cantrip). The number of stages are because eventually if the player roleplayed hard enough and did some mini campaign length tasks for the patron the blade would gaun enough power to become an artifact weapon. I am insteressted to see what people think.

The Shard of Otiax, The Key to the Gate   

Legend: Otiax is a bit of a conundrum because it seems to have originated outside the known cosmology of the planes. A few sources of pact magic lore refer to some plane or place called the Far Realm, but most offer no explanation of Otiax’s past. Some even posit that Otiax is somehow the key to reaching the Far Realm, but that supposition has more to do with Otiax’s appearance than with any real evidence of its nature. Because Otiax never speaks, it can shed no light on the issue. Binding with it is more a matter of instinct and will than of deliberation. This weapon or born of a shard of its form entering the material realm and has been passed from many hands its power lost but its mind dormant waiting for one worthy.

Dreams: When Otiax manifests in the dreams of the warlock they appear as a locked golden gate appears within its seal. Blue fog curls out in wispy tendrils from between the bars, obscuring what lies beyond. …………..After a moment of silence, some unseen force crashes against the barrier. Then the gate shakes and rattles loudly, as though some creature is desperate to open it…………….. Ragged breathing becomes audible, and the fog swirls around some indistinct yet terrible form…………... At last the raging stops, and the azure vapor passes through the gate………….. The sound of the tumblers turning in the lock becomes audible, then the gate creaks open.

Minor Displays: At certain points the blade will be surrounded by thin wisps of light blue fog even in the strongest wind. Also if ever the voice within tries to speak to the warlock they will commune with them in this form.

Mindset: Otiax’s motives remain a mystery, but its influence is clear. When confronted with unopened doors or gates, it will become agitated and nervous, pressing the wielder to open the locked door/gate. This emotional state lasts until the door or gate is opened, or until they can no longer see it. Furthermore, Otiax cannot abide a lock remaining secured. Thus, whenever the wielder sees a key, Otiax will press them to opening the corresponding lock.

Stage 1: the weapon appears as a lifeless silver longsword

Stage 2 Awakened: Your weapon becomes a +1 weapon. Open Portal. As a bonus action, you can open or close (your choice) a door, chest, box, window, bag, pouch, bottle, barrel, or other container within 5 feet of you. If anything resists this activity (such as a bar on a door or a lock on a chest), the effect fails. In addition, the effect can only open and close things weighing 30 pounds or less. Thus, doors, chests, and similar objects sized for enormous creatures may be beyond this effect’s ability to affect.

Stage 3 Empowered: You may curse two targets at a time with your Hexblade’s Curse ability. Unlock. As a major action, you can unlock a single lock that you can touch, provided that its DC is less than or equal to your warlock level. This ability grants you no protection from any traps that might be associated with the lock. Once you have used this ability, you cannot do so again until you have completed a short rest.

Stage 4 Masterpiece: Combat Reflexes. You may make additional opportunity attacks using this weapon equal to half your Dex modifier rounded down (minimum of 1).

Stage 5 Exalted: The Unlock ability now works on DCs less than or equal to twice your warlock level. Concealing Mist. You can whip the mist that constantly surrounds you into a concealing screen that grants you the ability to use the Armor of Hexes ability on all opponents in the encounter including those you have not cursed. Once you have used this ability you cannot use it again until you have completed a long rest.

Stage 6 Legendary: Your +1 bonus becomes +3.  You gain one use of the Gate spell once per long rest.

SPECIAL Stage - Pact of Blade Chosen: The weapon you summon must be this weapon and cannot be changed. Its appearance changes to fit its origin forming into both the shape of a key and taking on the appearance of a blade of the far realm. It can change the form the weapon takes much like a standard pact of the blade item, though it must be either a longsword, dagger or greatsword.

SPECIAL Stage 2 Curse Bringer Invocation Chosen: This weapon becomes set once more into the form of a greatsword gaining the ability of the invocation and allows you to ignore the no two-handed restriction on the hex warrior ability when using this blade.

Weapons Sentience

Stage 1: n/a

Stage 2: INT 10 (+0) WIS 10 (+0) CHA 5 (-3)

Stage 3: INT 13 (+1) WIS 13 (+1) CHA 9 (-1)

Stage 4: INT 14 (+2) WIS 16 (+3) CHA 14 (+2)

Stage 5: INT 18 (+4) WIS 20 (+5) CHA 18 (+4)

Stage 6: INT 25 (+7) WIS 25 (+7) CHA 20 (+5)

Conflict occurs if wielders actions do not conform to the restrictions provided in the mindset.

From Stage 3 the blade may attempt control of the wielder.

If a sentient item attempts to take control of its wielder, the wielder must make a Charisma Saving Throw, with a DC equal to 12 + the item’s Charisma modifier. On a failed save, the wielder is Charmed by the item for 1d12 hours. While Charmed, the wielder must try to follow the item’s commands. If the wielder takes damage, it can repeat the Saving Throw, ending the effect on a success. Whether the attempt to control its user succeeds or fails, the item can’t use this power again until the next dawn.

I will also admit the weapon has been slightly designed with the player in mind hence why certain abilties only work for the hexblade warlock and why certain abilties are really specific. If it were a different campaign and player/character i would probably not have this exact weapon.