The Core is one of the final areas of Undertale, following on from exploring the Hotlands.
Follow the shadowy figures into the core and Alphys will give you some directions to follow.
After a few false turns you'll encounter some lasers - Alphys will get the order wrong, and they'll actually be blue, blue, orange so let the first two pass through you then run east through the last one.
Keep following Alphys's directions. She'll continue to lead you the wrong way, but you still need to do what she tells you.
When you reach the long conveyor belt the lasers will turn on and off at random.
Run to the right when they're off, and when the screen flashes stand still and wait for them to cycle on and off again.
At the end head north, use the save point, then head north and east until you find a barrier blocking the exit to the right. From here head north as far as you can, then east across the bridge to find the switch that disables the barrier. Double back to where it was and head east again.
You'll come to an area containing a door, a save point, and an elevator that will take you back to the entrance of The Core. Use the save point, head back to resupply if you need it, and then go through the door.
Mettaton EX boss fight
When the fight begins you'll be in your yellow form but won't be able to damage Mettaton at all. Your shots will work against his attacks, though, so take a defensive stance for a few turns until Alphys calls, then use Turn Around from the Act menu and flip the switch on his back.
He'll now take on a new form. You still can't damage him, and instead need to boost your Ratings. Pose will give you a small boost, Boast will give you a larger boost if you manage to avoid getting hit by his attacks, and Heel Turn will give you a boost every time you take damage, but unless you're confident that you can avoid getting hit we'd suggest sticking to Pose.
Your Ratings will drop slowly when it's your turn, so choose your actions as quickly as possible.
He has a variety of attacks to call on:
- Blocks and bombs: Shoot the bombs as they fall to destroy the blocks.
- Falling Arms: Shoot the yellow blocks to destroy the arms; hitting anywhere else will cause the arms to fall faster, so precision is vital.
- Disco Ball: Shoot the ball to turn the beams blue, then keep still.
Once your Ratings reach 10,000 he'll spare you.
After the fight head north; Alphys will follow you and talk to you a few times. Use the elevator to leave The Core.
It's all a bit drab here, and none of the save points will refill your HP. Follow the long path through the greyness, ignore the elevator, and you'll eventually arrive at a familiar-yet-different location.
Explore the house thoroughly to learn more about what's been going on, and grab the keys from the kitchen and outside the bedroom before heading down to the basement.
Playing Undertale? Our complete Undertale walkthrough and Pacifist guide will explain how to finish the game while sparing everyone, covering all areas including the Ruins, Snowdin, Waterfall, Hotlands and The Core and more. You can also learn about how to unlock every Undertale ending and all Undertale Trophies, access hard mode and play the game in the most evil way possible with a Genocide run.
More exposition is available down here, and when you reach the end you'll find a save point and an elevator that can take you back to the start of the area. Save, ignore the elevator for now, and head down and then east to meet up with Sans again.
He'll now judge you based on your performance thus far, and then tell you you have two options: kill Asgore, or let him kill you.
Optional: If you want more plot details, quit the game, reload it, talk to Sans again, and after repeating the previous conversation he'll give you a secret password. Quit and reload a second time, talk to him again, and then quit and reload once more; Sans will now give you the key to his bedroom, and if you head back there and poke around you can find out more about him.
Head to the far end of the corridor, head north, make sure you have the Snowman Piece and the Butterscotch Pie in your inventory, and use the save point.
Keep heading up and you'll meet Asgore - after the conversation follow him inside. When you reach the final door he'll give you the option to turn back; if there's anything else you want to do then now is the time to do it, as once you go through there's no turning back.