The Otherside Portal is the way into the Otherside, the dimension added by the Deeper and Darker mod, and it’s lit inside the big 
Reinforced Deepslate frame in the middle of every Ancient City. To light it you’ll need a 
Heart of the Deep, which the Warden drops when it’s killed, so getting through will mean beating the Warden first.
Without the mod, that frame is only decoration, which is why lighting it is usually the first thing players look up once Deeper and Darker is installed. The heart gets used up when the portal lights, and the portal works both ways, so it’ll bring you back to the Overworld once you’ve finished exploring.
Finding a Frame
The easiest frame to use is the one in the center of an Ancient City, which is a vanilla structure that generates deep underground in the Deep Dark biome. These cities are packed with 
Sculk Sensors and 
Sculk Shriekers, so it’s worth sneaking whenever you’re close to one, as the noise you make is what brings out the Warden.
The frame is the large archway of Reinforced Deepslate at the heart of the city, with an opening 20 blocks wide and 6 blocks tall. It’s empty in a new world, though it doesn’t stay that way for long, as Sculk tends to creep across it from the blocks around it.
Reinforced Deepslate can’t be mined or crafted in Survival, so you won’t be able to build a frame of your own outside Creative, where any opening from 2 to 21 blocks across will work. There’s also a smaller frame inside some Ancient Temples in the Otherside, and the mod builds one around every portal it makes for you on the other side.
Lighting the Portal
Lighting it comes down to three steps, and the only hard one is the Warden. Each step has to be done in order, as the heart won’t do anything until the opening is completely clear.
Kill the Warden
Heart of the Deep
The Warden is summoned by the Sculk Shriekers in an Ancient City, once they’ve been set off four times by noise, and it has 500 health, which equates to 250 hearts. Every Warden drops one Heart of the Deep along with 1 to 3 
Warden Carapace, so a single kill is all it takes to light the portal.
Clear the Opening
Nothing but air inside
The opening has to be completely empty before the portal will light, and anything inside it stops it working, like 
Sculk Veins growing along the inner edges, a Candle, or water. Sculk Veins are the usual culprit, so walk around the inside of the frame and break any you can see, as they’re easy to miss in the dark.
Sculk Veins break almost instantly by hand, though a Hoe is the right tool if you’d like to keep them with Silk Touch. Clearing the whole frame only takes a minute, and it’s worth staying quiet while you do it, as the city’s sensors are still listening.
Use the Heart on the Frame
Inside face of the frame
Hold the Heart of the Deep and use it on the inside face of the frame, like the top of the bottom row, so the space in front of where you click is inside the opening. The portal will fill the whole frame with the sound of a Sculk Catalyst blooming, and the heart is used up, unless you’re in Creative.
A Heart of the Deep will only light a frame in the Overworld or in the Otherside, so a frame built in the Nether or the End won’t do anything. Some older guides also say the heart is kept after use, but it’s used up now, so each new portal costs another Warden.
If nothing happens, it’s almost always something left inside the opening, or a click on the outside of the frame instead of the inside. Frames you’ve built in Creative also need all four sides to be Reinforced Deepslate, as the corners don’t matter but every other edge block does.
Traveling Through
The portal works just like a Nether Portal, so you’ll only need to stand in it for a few seconds and the screen will fade as you’re pulled through. The Otherside uses the same coordinates as the Overworld, which means the spot you arrive at lines up with your portal above, rather than being divided by 8 like the Nether.
Your Return Portal
When you arrive, the mod looks for an existing Otherside Portal within 128 blocks, and if there isn’t one it builds a new frame of Reinforced Deepslate with a 6 by 3 opening. It’ll try to find a cave with enough headroom first, and if there’s no room at all, it clears out a space and puts down a floor of Reinforced Deepslate for you to stand on.
The Otherside is a dark cave world between a bedrock floor and a bedrock ceiling, so your new portal will usually be underground in one of its four biomes. There’s a lot more on what you’ll find there, and what to bring, on its own page.
Getting Back
Stepping back through the portal you arrived in will take you home, and the mod links it to the closest Otherside Portal within 128 blocks of the same spot in the Overworld. Since the return frame is made of Reinforced Deepslate, it can’t be broken by mobs or explosions, so it’ll still be there whenever you come back to it.
If You Lose the Portal
It’s worth noting down the portal’s coordinates before you wander off, as the caves all start to look the same after a while. The 
Ancient Compass won’t help you here either, as it points to the nearest Ancient Temple rather than to your portal.
Beds work in the Otherside, so placing one near your portal gives you somewhere to respawn if things go wrong. Respawn Anchors don’t work there, though, so a Bed is the only way to set a spawn point in the dimension.
If you do lose the portal, dying will always send you back to your spawn in the Overworld, as long as you haven’t slept in a Bed in the Otherside. That’s not a great way to get home with a full inventory, which is another reason to keep the coordinates written down.
FAQ
With Deeper and Darker installed, you’ll need a Heart of the Deep, which the Warden always drops. Clear any Sculk Veins out of the opening, then use the heart on the inside face of the Reinforced Deepslate frame and it will light, leading to the Otherside.
The frame in an Ancient City is only decoration in plain Minecraft, as there’s no item that lights it. It’s mods like Deeper and Darker that turn it into a working portal, which is why the Heart of the Deep is needed to open it.
Lighting a portal uses up the heart, unless you’re playing in Creative. You’ll need to kill another Warden for each extra portal, and for the Sonorous Staff as well, since that’s crafted with a heart too.
Something is usually still inside the opening, like Sculk Veins along the edges, a Candle or water, as it needs to be completely empty. It’ll also only light in the Overworld or the Otherside, and you’ll need to click the inside face of the frame rather than the outside.
