Your armor becomes vibrant while concentrating on this spell, and each round after the first your armor begins to pulsate more violently. Upon reaching the detonation round your armor will begin to audibly chatter against itself if it is made of metal, but loose clothing will violently sway. This effect is noticeable to anyone within 10 feet of the caster. The caster must maintain concentration on the spell for the duration of all rounds in order for targets to suffer all effects. If the concentration is interrupted the spell is lost with no effect.
During the round of casting, base damage is 3D8 thunder damage and gains 1D8 thunder damage each round until the detonation round (5D8 for a level 3 spell). Upon detonation, creatures within a 15 ft cube centered on the caster must make a Constitution saving throw. On a failed constitution save during the detonation round targets will take the full damage of the detonation and become deafened for the round, and must make a constitution save at the start of each of their turns or remain deafened until a successful save. On a successful constitution saving throw, creatures take half damage and are not deafened. The spellcaster may detonate the spell early, but will only trigger the damage based on the current round and will not deafen the creatures. Detonation must be made during the spellcasters turn but can be done at any point during the spellcaster's turn as a free action.
In addition, the force of Thunder Bomb pushes unsecured objects and debris in the area of effect 10 feet away from you while emitting a thunderous boom audible out to 500 feet. The detonation is so intense that it can be felt 100 feet out as a ground tremor.
At Higher Levels. When you cast this spell using a spell slot of 4 or higher you may extend the duration by one round for each slot above 3rd. If the spellcaster decides to extend the duration they must maintain concentration until detonation round or creatures will not suffer the deafened effect.
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