The Mourning Blade

This is a magic item. It's also an experiment with magic weapons, where they start out with minor effects but "level up" as you use them to gain cooler powers.

Source: rockydesign on cgtrader

The Mourning Blade is an artifact of the Mourn family, a lineage of elven spellswords. It is usually kept locked away, for it is only given to those who have truly learned the price for taking a life. The hope is that, once you have it, you never use it.

Of course, the people who wind up finding it probably aren't of the Mourn lineage. For them, it's just  a Longsword +0 (counts as magical for overcoming resistance/immunity) that can shine light out of its eye, as per the light cantrip (or, lacking that, a torch). You may activate or deactivate this ability as a bonus action. You may only use this feature for an hour, after which this ability is disabled for 24 hours. When this also happens, it also cries a quicksilver tear.

Quicksilver tears restore a spell slot when consumed. The level of the spell slot is your choice. When you do this, you take 1d6 poison damage per level of spell slot restored. Against non-magic users, it is just a poison that deals 3d6 damage.

The first time you kill an enemy with the Mourning Blade, it cries a quicksilver tear. This will drip down onto your skin, automatically administering the effects to you. It also turns your eyes the same color as the sword's eye.

The second time you kill an enemy with the Mourning Blade, and every time afterwards, you cry a quicksilver tear. You may consume this tear on the same turn as the killing blow as a free action.

The third time you kill an enemy with the Mourning Blade, and every time afterwards, it becomes a dancing weapon that will automatically attack any creature that comes within 10 feet of the newly-made corpse. You have until the end of your next turn to get out of the area, or the sword attacks you. You cannot wield the weapon again until combat ends.