|This once placid myconid is now officially Up To No Good.|
Any monstrous spider or fungus can be affected. And sure, yeah, there are not a ton of fungal monsters in the monster manual, but the internet has some that you might like.
Once one of the eligible creature types has been irradiated with a dose of unlight, it develops a result from the table below over the course of a day or so. All effects are permanent, and multiple exposures will not result in further change. Also, you may want to give groups of identical abilities off here to keep things simple.
Tenebrous Mutation Table (d10)
1: The creature's intelligence increases to 20 and it's alignment changes to evil.
2: The creature exudes a shroud of inky spores in a 15-foot radius similar to a deeper darkness spell. The spores do not inhibit fungal or arachnid creatures.
3: Any creature struck by the creature's melee attack has their speed reduced to 2 until the end of their next turn as strands of semisolid darkness cling to them and inhibit their movement.
4: The creature's size increases in size until it is huge (10 feet by 10 feet). It gains +50 max hit points and automatically succeeds on the first saving throw it rolls, each encounter.
5: The creature can pass effortlessly through walls and doors of less than a foot in thickness unless they incorporate significant quantities of lead, gold, or another heavy metal.
6: The creature is invisible during the first round of each combat. This isn't an all the time thing, it's more like the unlight kicks in when it gets agitated enough for a fight.
7: The creature permanently bilocates. It occupies two spaces, but is the same creature. Damaging one damages the other, et ceteras. The two creatures can travel any distance apart but will be pulled into other planes together if one passes through a portal or plane shifts.
8: The creature gains resistance (half damage) against spell damage. It can also exhale a 5-foot wide, 25-foot long breath weapon of disintegrating darkness, inflicting 5d6 disintegration damage, Reflex half. It can do this three times per day but not two rounds in a row.
9: The creature grows a horrid mouth full of inky teeth in an unlikely location on it's body. The mouth makes an extra attack with the creature's normal attack bonus, each turn, inflicting something like 2d8 damage on a hit.
10: The creature sprouts a set of asymmetrical wings capable of propelling it through air, water, or interstellar void. It also develops the ability to reproduce via fission and to digest any organic material, if it did not possess those abilities before.