The wand of wonder item states the following (emphasis mine):
If the effect causes you to cast a spell from the wand, the spell's save DC is 15. If the spell normally has a range expressed in feet, its range becomes 120 feet if it isn't already.
If an effect covers an area, you must center the spell on and include the target. If an effect has multiple possible subjects, the DM randomly determines which ones are affected.
There are 6 spells that can be cast from the wand that can possibly affect more than one target in an area:
- faerie fire
- gust of wind
- lightning bolt
- stinking cloud
The slow spell affects up to 6 creatures of your choice in the cube, and all of the other spells affect all targets in the area (so all creatures in a line for lightning bolt, all creatures in a sphere for fireball, etc.).
How does this interact with the wand of wonder description? It's clear that if you target a creature with the wand, then that creature must be affected by the spell cast by the wand. Does the sentence "if an effect has multiple possible subjects..." mean that even for spells like fireball, the DM has to roll randomly to determine if a particular target is affected?
So suppose I use the wand on a creature and I am forced to cast fireball (centered on it), and suppose I have an ally right next to the targeted creature. Does my ally automatically get hit by fireball or does the DM determine randomly if the ally is affected?
I'm confused about the wording because the slow spell has caster choice determine the targets, and so that mechanically operates differently from fireball. So I can understand if the random determination of targets was intended only to apply to a spell like slow and not to something that is indiscriminate like fireball.
Edit: My other confusion rises from the use of the word "subject". If the sentence said "multiple possible targets", I think it would be clearer to me.