They drop from a load of monsters:
Metal heads/Dangerous metal heads
Hot heads (in the volcano)
Squid kids/Dangerous squid kids
The squid monsters
Iridium bats
Mummies (probably the best way to farm them)
Ghosts
Haunted skulls
That's a lot of different monsters, of which only the squid kids being of any particular rarity.
You can also buy them from krobus and from sunfish fish ponds I think
On top of that, none of the recipes using it are really bulk craft recipes (bar maybe bombs and everyone is just buying them later on anyway)
So I honestly think it's pretty balanced
I agree that Solar Essence is more useful, but Void Essence is easier to obtain. Adding more monsters that drop Solar Essence while reducing Void Essence drops could balance things better. Also, replacing Solar Essence with coal in Mega Bomb recipes makes sense since other bombs use ore and coal. Writing essays has always been a struggle for me, so I decided to try
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