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

{
    "display": {
        "icon": "minecraft:flint_and_steel",
        "title": "Enter the Nether"
    },
    "parent": "minecraft:story/form_obsidian",
    "criteria": {
        "entered_nether_portal": {
            "trigger": "minecraft:enter_block",
            "conditions": {
                "block": "minecraft:portal"
            }
        }
    }
}