Void Bubble

Making a circle with her hand the mage blew a small bubble. Floating slowly around it looks harmless, this strange act by the mage causing laughter throughout the bandits attacking. One even walk up to it and popped it, before having his arm blown off with a loud bang.

The Void Bubble spell creates a bubble of empty space held together with magic. Upon creation the bubble will float in a random direction for 5 feet each turn (unless blown by a spell or wind) the direction of which will be determined by 1d4 (or by DM's discretion). This bubble is completely harmless and normal to the naked eye. However, if popped or triggered by the caster the bubble will collapse and the matter around the bubble will rush to fill the void that was made. This causes a small explosion (dealing 2d8 force damage) to anyone within 5 feet of the bubble. The bubble will always stay within a 30 foot radius of the caster however, to the best of it's ability. should the bubble be forced to leave the radius it will immediately pop. 

The bubble has 1hp regardless of size and will only move 5ft each turn.

This spell can be upcasted.
For each spell slot above level 1 used, another bubble
can be created or an existing bubble can grow
larger (adding 1d8 force damage each spell level and
5ft of explosion radius every 2 spell level, up to level nine)