custom_mc_advancements/data/canopy/advancements/mobs.json

{
    "display": {
        "icon": {
            "item": "minecraft:zombie_head"
        },
        "title": "Mobs",
        "description": "Gotta kill 'em all!",
        "frame": "task",
        "show_toast": false,
        "announce_to_chat": false
    },
    "parent": "canopy:root",
    "criteria": {
        "kill_any_mob": {
            "trigger": "minecraft:player_killed_entity"
        }
    }
}