Fog Cloud plus a lightning spell also has a special effect.
What exactly is supposed to happen here, because I've immediately started to test it and I'm experiencing nothing noteworthy?
It’s a while since I tried it, but I’m pretty sure it used to be the case that the cloud became electrified. Does that not happen?
Can't say it did, no
As for Plant Growth + Fire I can't verify, as I don't have a character that can create Plant Growth, but the Druid spell Entangle creates Twisting Vines which can also be set ablaze with Fire.
