Magical items are not always universally usable by any creature that happens to pick it up and wave it about. Some require that the user be a certain race or class. Some require the user to spend time getting attuned to the item.
The fifth edition Dungeon Master’s Guide states: “Attuning to an item requires a creature to spend a short rest focused on only that item while being in physical contact with it” and “at the end of the short rest, a creature gains an intuitive understanding of how to activate any magical properties of the item” (p.138). A short rest is at least one hour of uninterrupted downtime.
It seemed to me that it should take more than an hour of staring for a lucky Paladin to unlock the mysteries of the longsword (a Holy Avenger) she just picked up. I thought that the attunement process should include a little more effort on the part of the bearer. Another problem I faced was that a Holy Avenger was a bit over powered for a second or third level Paladin.
To solve both of these problems I decided that the power of the magic item could grow over time, becoming something that could remain with a character for a lifetime, essentially leveling up along with the character. This is no longer attunement, it is so much more…
At first, the sword appeared to be a simple [+1] magical sword, a great find for a young [lower level] Paladin. After some use, the sword decided that she was a worthy bearer. During a skirmish with Goblins, the Paladin landed a killing blow. She was immediately filled with a great sense of satisfaction for ridding the world of such an evil creature. Unperceived by the Paladin, the sword had tasked her with killing three evil creatures in three separate encounters. This goblin was the first.
The Paladin seemed a little concerned by the outcome and recognized that the strong positive emotions over killing the creature originated with the sword. She was hesitant to use it for a time, preferring to use her javelins and allowing her companions to finish off their enemies. [DM allows that could just have been poor luck with the dice.]
Eventually, however, she landed another killing blow on a deserving evil goblin or hobgoblin. That sense of immense satisfaction returned to her again, much stronger this time. The thirsty sword seemed eager to take another evil life and it wasn’t long before Paladin finally fulfilled her task.
The sword rewarded the Paladin by increasing its magical power [to +3] and she understood that fiends and undead would suffer additional [radiant] damage from an attack. Any evil creature would suffer [psychic] damage simply by being in contact with the weapon for a short amount of time.
The sword has only begun to reveal itself; the Paladin still has much to learn.