minecraft-assets/assets/minecraft/advancements/nether/find_fortress.json

{
    "display": {
        "icon": {
            "item": "minecraft:nether_brick"
        },
        "title": "A Terrible Fortress",
        "description": "Break your way into a Nether Fortress"
    },
    "parent": "minecraft:nether/root",
    "criteria": {
        "fortress": {
            "trigger": "minecraft:location",
            "conditions": {
                "feature": "Fortress"
            }
        }
    }
}