minecraft-assets/assets/minecraft/advancements/end/respawn_dragon.json

{
    "display": {
        "icon": {
            "item": "minecraft:end_crystal"
        },
        "title": "The End... Again...",
        "description": "Respawn the ender dragon",
        "frame": "goal"
    },
    "parent": "end/kill_dragon",
    "criteria": {
        "summoned_dragon": {
            "trigger": "minecraft:summoned_entity",
            "conditions": {
                "entity": {
                    "type": "ender_dragon"
                }
            }
        }
    }
}