The only way this Pact makes any sense to me is through multi-classing into it once you have found one of these powerful weapons. If you are adventuring and come across a powerful sentience weapon, it promises you more power if you "give yourself to it". You then use the weapon to tap into new abilities that it grants you. This would then manifest as multi-classing as a Hexblade.
Heirloom.
Gift from previous wielder.
Thing you created yourself, with epic and one of a kind materials, in a once in a life time Time and place, under super natural circumstances. This combines well with either of the two before it, or both.
Lucky/fated discovery.
The thing called to you in your dreams and when you woke, a voice guided you to it, through various dangers and obstacles, into supernatural realms and with the aid of strange creatures, until finally you stood in the quiet place where the artifact rested, and when you laid your hands upon it you, and your fate, were irrevocably altered. You hope for good, but fear it is for ill.
It was a regular weapon, and you a regular person, until [event], and then you and the sword (or flail or stick or whatever) were irrevocably blah blah see above. (See, the princes of Leah, in the Shannara series, and how he Sword of Leah became magical, and in becoming so, changed its weilders as well.
Literally a basic magic item that is *gaining* power as you do.
I'm sure others can think of more, and each of those has many variations within them, not to mention combinations. Every one can be a level 1 concept, and some only make sense as a level 1 concept.