If your egg has been battling for ages and is still an egg, one of the evolution requirements is usually missing. Every legendary evolution in this pack needs both a minimum level and the correct biome.
Checklist
1. Check the level.
- Arcegg needs Level 32 to become Arcdeo.
- Arcdeo needs Level 65 to become Arceus.
- Every other legendary egg needs Level 65.
2. Check the location. Press F3 and read the biome name on the debug screen.
- Arcegg, Arcdeo, EterEgg, DialgaEgg, PalkiaEgg, GiratinaEgg and MewtwoEgg must be in The End.
- The Overworld eggs must be in the same biome they spawn in. For example, ZygarEgg must be in a Swamp, ArticunoEgg in Ice Spikes, and ZapdosEgg in a Cherry Grove.
3. Watch the border. A biome can change within a few blocks. If you are near the edge, walk deeper into the correct biome before trying again.
4. Remember Arceus has two stages. An Arcegg at Level 65 still has to become Arcdeo first.
Using /checkspawn to confirm the biome
If you are unsure whether you are in the right place, run /checkspawn. With Cobblemon, this command shows the monsters available in your current biome. If the matching legendary egg appears in that list, you are standing in its native biome. That is exactly where it can evolve.
Once both requirements are met, keep training the egg in that biome until it evolves.