minecraft-assets/assets/minecraft/advancements/story/summon_wither.json

{
    "display": {
        "icon": "minecraft:nether_star",
        "title": "Summon the Wither"
    },
    "parent": "minecraft:story/enter_the_nether",
    "criteria": {
        "summoned": {
            "trigger": "minecraft:summoned_entity",
            "conditions": {
                "entity": {
                    "type": "ender_dragon"
                }
            }
        }
    }
}