custom_mc_advancements/data/canopy/advancements/kill_a_warden.json

{
    "display": {
        "icon": {
	    "item": "minecraft:sculk_catalyst"
        },
        "title": "Mojang... is that the best you've got?",
        "description": "Kill a Warden. So much for sneaking.",
        "frame": "challenge",
        "announce_to_chat": true
    },
    "parent": "canopy:mobs",
    "criteria": {
        "kill_warden": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {
                "entity": {
		    "type": "minecraft:warden"
                }
            }
        }
    }
}