The Heart of the Deep is an item from the Deeper and Darker mod that drops from the Warden every time it’s killed, and it’s what opens the portal to the Otherside. You’ll use it on the empty portal frame in the middle of an Ancient City, which lights up the portal and uses the Heart up.
It’s also the main ingredient in the 
Sonorous Staff, so you’ll need two of them if you want the staff as well as your own portal. Each one only stacks to 1, and it won’t burn up if you drop it in Lava, which is useful given where you’ll be fighting for it.
Obtaining
Every Warden drops one Heart of the Deep when it dies, along with 1 to 3 
Warden Carapace. It’s the only way to get one, as it doesn’t turn up in any chest, so you’ll need to kill a Warden to get into the Otherside at all.
A Warden appears when a Sculk Shrieker in an Ancient City goes off too many times, so you’re able to call one on purpose by setting off the same shrieker a few times. It has 500 health and a sonic boom that goes through armor, so it’s worth fighting it from somewhere it can’t reach you, or trapping it before you start.
If you’re on a server and someone has already opened a portal, you can skip the fight, as anyone is able to walk through a lit portal. You’ll still want a Heart for the Sonorous Staff later, though, so it’s worth killing a Warden once you’ve got the gear for it.
Uses
Opening the Otherside Portal
The middle of every Ancient City has a large empty frame made of 
Reinforced Deepslate. Right-click inside it with the Heart of the Deep and the portal will light up, which uses the Heart up unless you’re in Creative mode.
It works on any frame of Reinforced Deepslate between 2 and 21 blocks across on the inside, but you’ll only find the block in Ancient Cities, as it doesn’t drop anything when it’s broken in Survival. The Otherside Portal page covers how the portal works once it’s lit and what happens on the other side.
The Sonorous Staff
The Heart of the Deep goes in the top corner of the Sonorous Staff recipe, with 2 
Soul Crystals and 2 
Sculk Bones. The staff fires a sonic boom like the Warden’s, which deals up to 50 damage at full charge and goes straight through armor.
Those are the only two uses for it, so once you’ve opened a portal and made the staff, any spare Hearts are only useful for opening more portals. That can still be handy if you’d like a second way into the Otherside from another Ancient City.
The Heartbeat
While you’re carrying a Heart of the Deep, you’ll hear a heartbeat now and then, the same sound the Warden makes. It doesn’t mean anything is nearby, as it plays at random whatever you’re doing, and it’s only there to make the item feel alive.
Server owners are able to turn the sound off in the mod’s config with wardenHeartPulses. Older guides say the heartbeat points you toward structures, but there’s nothing in the mod that does that, so you don’t need to follow it.
FAQ
The Heart of the Deep comes from the Deeper and Darker mod, and the Warden drops one every time it’s killed. It isn’t found in any chest, so you’ll have to summon and kill a Warden in an Ancient City, which you can do by setting off a Sculk Shrieker a few times.
It opens the Otherside Portal when you use it inside the Reinforced Deepslate frame in the middle of an Ancient City, and it’s used up when you do. It’s also crafted into the Sonorous Staff with 2 Soul Crystals and 2 Sculk Bones.
Hold it and right-click the inside of the big Reinforced Deepslate frame at the center of an Ancient City, and the portal will light up. You’ll need to be in the Overworld or the Otherside for it to work, and the frame has to be complete.
The heartbeat plays at random while it’s in your inventory, the same sound the Warden makes, and it doesn’t point you anywhere. It can be turned off in the mod’s config with the wardenHeartPulses setting if it gets annoying.
