DnD 5E: Magic item for one final defiance at death

I’m playing a fighter in a pretty standard D&D 5th edition game. Currently 7th level. I’m wanting to ask my DM for a magic item that would be something like the following. I’ve tried to make it something that would let me go out with one heroic bang (and also give one of my backup characters a chance to shine…) without making it something that is "always on", hence the drawback if it’s not used. I’m also trying to make this a conscious decision as one final act of defiance. Perhaps it’s something given to soldiers of a certain army? Anyhow, I’m wondering if this is a reasonable item to ask for and if there exists anything similar to this that already exists rather than being goofy homebrew of an inexperienced player. And if there is anything that can be done to improve it, mechanically or wording-wise.

Bead of Final Countdown:

When a creature is locked in mortal combat, it may become apparent that this will be its last such encounter. The creature can use a bonus action to place this bead in its mouth, biting down on it. This begins to charge a delayed blast fireball. When the creature dies (not just unconscious, but truly dead) the fireball explodes as in the spell, dealing damage to all creatures in range and destroying the creature’s body. If the creature does not die within two minutes, or if it removes the bead from its mouth, it suffers one level of exhaustion from the stress of holding such an explosion in its mouth.