Asriel Dreemurr is the true final boss of Undertale, coming after you've explored the bonus True Laboratory section.
If you're playing a Genocide run, however, you'll be fighting someone different in the final encounter - the prior link explains who. But whichever way you're playing, our wider Undertale guide delves into other areas of the game.
Asgore boss fight
All you can do during the first phase of the fight is select Hope or Dream from your Act menu.
Hope will reduce the damage you take in the next attack, and Dream will fill any empty inventory slots with healing items, so pick them appropriately to keep yourself alive as the battle progresses - dying isn't terminal in this battle and it's impossible to lose, but it's over much quicker if you can avoid taking too much damage.
Asriel has quite a range of attacks that you'll need to endure for a while:
- Star Blazing will rain down large stars that will split into smaller ones when they hit the box, and it's fairly easy to avoid.
- Chaos Blaster can be avoided by moving around while the guns aim at you, and then standing still when it fires. He may occasionally follow this up with a quick Star Blazing attack.
- Shocker Breaker is very hard to avoid - boxes with exclamation marks will appear at the bottom of the combat area, and you'll need to move to the left or right to get out of the way quickly.
- The box will occasionally shrink and he'll unleash his swords. Just move in the opposite direction to him to avoid them.
After several rounds of this he'll attempt to suck you into a vortex - keep moving down to avoid being consumed by it.
When the next phase begins your only option on your turn will be Struggle, and he'll attack with some colourful beams that curve in from either side - these are best avoided by hugging the top of the box and moving left and right to dodge them.
After a few rounds of this your Act menu will turn into a Save menu, and from within it you'll be able to summon some of your allies.
- Undyne will stick to her arrow attacks, so just block them as before. Use any three of the Act options to save her.
- Alphys fights in a similar manner to Mettaton - shoot the bombs to break the blocks, and use any three Acts to save her.
- Papyrus and Sans come as a pair, and their attacks are the same as your first fight with Papyrus. You need to use four Acts to save them.
- Toriel and Asgore use a variety of flame attacks - these are the hardest to deal with as they can use some combinations that leave you little room to dodge. Use four different Acts to save them.
The fight is now essentially over - Asriel will continue to attack, but no matter how hard he tries he cannot kill you and will eventually give up.
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.
After the Asgore boss fight
After the cutscene plays you're now free to leave the world, but before you do we'd strongly recommend talking to everyone one last time and, without spoiling specifics, walking all the way back to where you started (!) to fill in the rest of the story.
When you're ready, head through the barrier and watch the credits roll... until Asriel shows up. If you can now avoid hitting all the names in the credits you will be able to access the secret area in Snowdin during a subsequent playthrough.