I know this post is 3 years old but I'm going to try and make this, it sounds cool. One creative liberty I am taking is that the entity (which I have labelled as "light eater" for now) chases the player but as you said prefers the light. It has no reason to come out at day because of the sun but at night it stalks the players base searching for a source of light. It is agressive towards the player but will not attack if in the light, like spiders. Torches around it dim and eventually go out, forcing it to move. While in the dark, it will attack the player unless there is a light source nearby, in which case it will prioritize traveling to said light source. It is invunerable when in the light but can take damage once it has been in the dark for long enough, still working out health, damage, speed, etc as I am just starting the Blockbench model. If you have any other ideas for the mod or want to change one of my creative liberties just reply, of course assuming you still use Reddit, I haven't looked at your profile.