Dryad
Lice, or "Dryads" - as coined by the Twelfth Doctor, were insect-like creatures which lived in wood and fed on humans. They lived in a house owned by the Landlord after he found three individuals outside, which had some healing properties that saved Eliza, however turning her partially into wood. In order to properly heal someone however, they had to be regularly fed people, so the Landlord decided to make the house cheap to lure people into buying it. He'd also use a tuning fork to call them to the area where the inhabitants were.
In the end, Eliza controlled all the Dryads into eating her and her son in order to stop the killing that was keeping her alive, and as a result all the Dryads left. (TV: Knock Knock)
Biology
Dryads bore a great resemblance to cockroaches. They had a chitinous hide, were a little bigger and had a brownish red color. They also had biolumenescent antennae on their heads.
Given that the Landlord only found three to start with, only for them to become a gigantic swarm that infested the whole house, it is possible they reproduce asexually and at an incredible rate.
Abilities
The Dryads had incredible healing abilities and could preserve a person indefinitely by transmuting their body into a substance with the same color and texture as wood. They would however have to be fed in order to maintain this preserved state or the person would revert to their original state and die.
The Dryads' natural habitat was the element of wood which an entire swarm could inhabit.
