Is this "Bead of Final Countdown" homebrew magic item (for one final act of defiance in death) balanced?

Question

I'm playing a fighter in a pretty standard D&D 5th edition game. My character is currently 7th level. I'm wanting to ask my DM for a magic item that would be something like the following:

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 delayed blast fireball spell, dealing damage to all creatures in range and destroying the creature's body. If the creature that places the bead in its mouth 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.

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.

Answer 1

This is likely overpowered, but should work due to its conditions

First, there are a few things you need to consider in regards to the item:

1. What is the saving throw of the spell?¹ For context, at level 7, a spell caster who took standard array ability scores, and put ASI into their casting ability would would have a spell save DC of 15. A spellcaster who has done the same and would have access to Delayed Blast Fireball (13th level) would have a spell save DC of 18.
2. What happens to the damage after the first minute has passed?² Delayed Blast Fireball lasts for 1 minute, but this item you have suggested currently is set to last for two. After the first minute has passed, does the blast keep charging? This would mean increasing the maximum damage from 22d6 to 32d6, which averages to 96 damage.
3. What happens to the spell after the 2 minutes have passed?³ Does the item detonate after two minutes, even if the target is still alive? or is it as if the bead has been dispelled, and nothing takes place?
4. What happens to the bead after a successful or unsuccessful use?⁴ Can the bead be used by another player after exploding? Is the bead inert after first failed use? This should be clarified.
5. Can you move with the bead in your mouth?⁵ A major restriction of the spell is that once the bead is initially placed, it cannot be moved without making a dexterity saving throw. is this true for the creature with the bead in its mouth?

Second, as answered here in regards to over-leveled spells, giving players access to spells that are a higher level than they should have access to often can seriously tip the scales in there favor, however in this case their are a few things that shouldn't make this a problem:

1. Delayed Blast Fireball is just damage, though a ton of it. The DM will not have to think about balancing for powerful side effects (Such as Prismatic Spray), instead just decreasing the numbers should suffice if they find it overwhelming. Talk to your DM about what seems reasonable.
2. This blast only goes off if its user dies. This is a pretty drastic condition that seriously limits the times the item will be used, so I think it justifies some higher damage than would be expected at level 7.
3. Exhaustion on a failed use. Exhaustion gets pretty devastating quickly, so this is certain to make a player need to be cautious about using this item (Strange that staying alive is failing).
4. The user's body is destroyed. At 7th level your Cleric/Druid/Healer only would have access to Revivify to bring a creature back from death, which does not work on destroyed bodies. Making this death a lot closer to permanent is an exciting consequence.

Overall, I think this item is close to workable. Work with your DM to answer the questions I've set, and you should have an item both of you are happy with. Or, see below for my take.

¹ With the amount of damage being dealt, even halving it would still hurt pretty hard. I think a DC 15 would be the best option, if not lower.
² Using the Quick Monster By Challenge Rating in the DMG (p.274), this would outright kill nearly any enemy CR2 or lower. At 7th level, most PCs would have less HP than this too. Assuming CON is +2 (likely all PCs, excluding fighters) most important stat, and average taken for every level.

Hit Die	"Average" HP/lvl	HP at level 7
d6	5.5	41
d8	6.5	49
d10	7.5	57
d12	8.5	65

It would seem this could pretty easily kill a Sorcerer or Wizard on a failed save, and if you are in cramped enough quarters knock out your entire party! I think having the damage charge for 1 minute, then hold at max charge for an extra minute would work well for the level you are asking for this item.
³ The bead should be rendered inert after 2 minutes have passed.
⁴ After detonating, the bead is lost. If the beads effect fails, it can be reused. I would also add that it cannot be used again until the following dawn, but you may like the gamble of stacking exhaustion.
⁵ Half movement speed, as the creature must move carefully with a bomb in there mouth.

Answer 2

This is all upside and no real downside

You are looking for an item that you only want to use when you think you are going to die, so really you want to make sure that you ONLY use it in that situation, but one level of exhaustion is such a small penalty that you might as well use this all the time.

Firstly exhaustion only effects skills, so for a class such as a fighter that is sometimes hardly even noticeable.

Secondly exhaustion can be easily cured by having a long rest, and most difficult fights are likely to be followed by a long rest anyway. It takes a pretty sadistic DM to run the party through a fight that almost kills someone and then do it again without letting the party rest.

Other missing things

There are a few missing items from the spell description such as spell DC; why it is 2 mins instead of 1 min, and what happens in the second minute; if the bead is reusable, either once it has exploded or not; is it movable with or without a save, and what DC that would be; is there friendly fire from it (I would hope so); are there any other changes to the spell?

Worldbuilding considerations

If an item like this exists a rich enemy is going to feed it to all sorts of minions and send them against you.

Thoughts to balance it

You first need to get beyond the worldbuilding considerations (maybe invent it yourself?) and beyond that your basic idea makes sense - benefit with a downside - but your downside needs to be comparable to the benefits of a 7th level spell.

You need to think of something that really makes you think "I am only going to use this when I am SURE I am going to die" rather than just when you think you might. This isn't the place to homebrew so I won't make any suggestions, but I would say if you survive then you need to be in a far worse place than if you survived without.

Alternative thoughts

The above notes are based on what seems to be your intention of an object you don't want to use, but you could just make it like a version of contingency and have no downside. Balance it as a normal magic item with cost and rarity and just let it be hanging around until the situation of your death arises (just don't forget to take it off when you reach old age and your family are around you tending to your failing health).