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

{
    "display": {
        "icon": {
            "item": "minecraft:end_stone"
        },
        "title": "The End",
        "description": "Or the beginning?",
        "background": "minecraft:textures/gui/advancements/backgrounds/end.png"
    },
    "criteria": {
        "entered_end_portal": {
            "trigger": "minecraft:enter_block",
            "conditions": {
                "block": "minecraft:end_portal"
            }
        }
    }
}