custom_mc_advancements/data/canopy/advancements/a_good_start.json

{
    "display": {
        "icon": {
	    "item": "minecraft:phantom_membrane"
        },
        "title": "A Good Start",
        "description": "Kill a phantom, hopefully more to come.",
        "frame": "task",
        "announce_to_chat": true
    },
    "parent": "canopy:mobs",
    "criteria": {
        "have_sed": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {
                "entity": {
		    "type": "minecraft:phantom"
                }
            }
        }
    }
}