About the mod:

This mod provides a configurable way to enforce players to remain underground after a set amount of time has passed. The gameplay provided by the default configurations follows as such:

Upon logging into a server for the first time or world creation, players have a grace period of 2 minutes to gather resources on the surface before heading underground.
Once the timer runs out, the player must remain below 64y, or sea level, and avoid any areas underground that have a view of the sky.
If the player is exposed to the hazard, they will receive 1 1/2 hearts of void damage every second.
During the night, surface damage will not be active, or if the timer is still present on players, the timer will be paused until morning.

If hosting a server, this mod is required on both the server and client

Configurable Options:

Countdown - the starting timer to count down for each player once they log in for the first time.
Damage height - the height at which players must remain below to avoid taking surface damage
Times per damage - the frequency with which the surface damage is dealt.
Damage amount - amount of surface damage dealt.
Damage type - type of damage the player takes. Can be VOID, FIRE, LAVA, DROWNING, SUFFOCATION or STARVING
Daytime tick - If the starting timer should tick down only during the daytime. At nighttime, the timer is paused. Surface damage will not happen at nighttime as well.
See sky damage - Surface damage will happen regardless of height, if the player is standing in a spot that can see the sky.

Vein Dweller