Gateways to Eternity is a Minecraft mod where you open giant portals which pour forth hordes of enemies in a series of waves, each wave stronger than the last. Completion of a wave will yield rewards, and completing the entire gateway will yield a completion bonus.
See the CurseForge Page for the full description.
For information on adding custom gateways, see the schema. A basic understanding of how to add datapack content is required.