What the hell kind of question is this?
It's like you're trying to do wordplay - Heavy Metal is only "Harder than Metal" semantically - if we're talking a specific style then "Heavy Metal" typically refers to the trad. stuff, which is arguably the least "hard" kind of metal around except like Glam Metal (depends on the band) or the absolute floweriest EuroPM. It's not rly a descriptor which evokes "harder," edgier stuff than "metal" by itself does, but if you're just making a word joke it would make sense.
But then you say "Djent" and even that stops making sense.
Someone said Extreme Metal, which is the correct (I GUESS???) answer. But basically, I can list off shit that's generally heavier than Heavy Metal proper is:
Death Metal, Black Metal, War Metal, Grindcore, early Industrial Metal (not including NIN & Neu Deutsch Harte or whatever it's called), Thrash Metal, Doom Metal, Sludge Metal, almost all Avant Garde Metal, Viking Metal, Speed Metal, Metallic Hardcore, probably honestly like 1/2 of Nu Metal (Slipknot and KoRn were listening to DM, and those downtuned riffs are heavy even if they suck), and… sure… probably most Djent, too. Oh, and Groove Metal probably but who gives a fuck about Groove Metal?