I'll bet you seasoned old players will know what I'm talking about when I spin you this yarn of a suggestion.

The Guardian Wisp is colored red and white, with a dark purple fire blade instead of an elementally colored one. This Wisp will always remain invincible, and will always be dormant.

Players who encounter this Wisp will notice that they have a key next to them, and are usually resting down a winding path with obstacles. Players will take the key to unlock a door that is blocking their path, doing so however will awaken the Guardian Wisp. This Wisp never stops moving when the key is held, and it will constantly chase the player holding the key. It is faster then a regular Wisp, and does not inflict elemental statuses. Instead, for every second that the Wisp is touching a player, it drains a single bar of life. T2 drains 2, and T3 drains 3.
However, one may think that you can simply set the key down, then pick it up multiple times to slowly but surely make your way to the gate.
The Guardian Wisp is wise to these tricks, so if the key is placed on the ground the wisp will hover over it. Forcing damage upon players who pick the key up. Taking damage from this Wisp will not make you drop whatever you are holding.

After it's key is used to unlock the gate, the Guardian Wisp explodes. Dropping only hearts on it's resting spot.

Can you guess what this is like? If you guess Phanto, from Super Mario Brothers 2, then you're right. These Wisps already look very similar to them, so I thought why not pay a bit of homage to them by having a key stealing scenario.