How can I respawn the Ender Dragon?
Minecraft 1.8 and lower
Most servers I've seen just reset the whole dimension by deleting the DIM1 (vanilla) or world_the_end (CraftBukkit) folder.
There is another way to respawn the Ender dragon, that will keep the player builds in The End, but it is a bit more complicated:
You need an NBT editor (like NBT Explorer). You need to open DIM1\r.0.0.mca and edit in Chunk [0, 0] the TerrainPopulated property to 0:
When the chunk is loaded again, the dragon will be re-created. Note that this will also re-create the obsidian tower(s) in that (0, 0) chunk. Also note that when you kill the new dragon, a new exit portal with a new dragon egg will be created – you may want to switch to creative to remove the unnecessary ones.
From version 1.9, the Ender dragon can be summoned in-game by placing four Ender Crystals on the edges of the exit portal pool, one on each side. More details on the Minecraft Wiki - Ender Dragon
Minecraft 1.9 and higher
To respawn the Ender Dragon, you must craft 4 End Crystals:
Place them around the End portal like so:
A respawning sequence will begin, refreshing the obsidian towers and original ender crystals, and then the dragon is spawned above the portal.
Use WorldEdit
It can be done from in game with the WorldEdit plugin (which most servers do), without needing to delete the -dim 1 folder, and therefore preserving any structure you built away from the main platform in the End.
It's rather easy:
- Go through the end portal.
//wand- Left click on one corner of the end platform
- Go to opposite corner (at least 200m away) and make a pile of blocks roughly 20 high
- Right click top block on your pile
//regen
And poof, everything in that entire area is as it original was. Hurry and /spawn before the dragon can generate and it wont even be there until a new player comes through the portal.