Time Bomb

You attempt to briefly attach a living time bomb to a creature. Make a ranged spell attack against a creature you can see within range. On a hit, an invisible time bomb is attached to that creature, becoming  more powerful the longer it is in effect. At any point in time during your round, you may have the bomb detonate using your bonus action, causing the creature to suffer 2d6+(1d6 at the beginning of each of your turns while the spell is in effect) points of Fire damage. The spell has no effect if it is dispelled early.

For each level of spell slot used to cast this spell beyond 2nd, the duration increases by 1 round and the initial damage increases by 1d6. For every 2 levels of spell slots used to cast this spell beyond 3rd, you may target an additional creature.