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

{
    "display": {
        "icon": {
            "item": "minecraft:skull",
            "data": 5
        },
        "title": "Free the End",
        "description": "Good luck"
    },
    "parent": "end/root",
    "criteria": {
        "killed_dragon": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {
                "entity": {
                    "type": "ender_dragon"
                }
            }
        }
    }
}