Of all the many spirit ashes in Elden Ring, the Mimic Tear is the only one that gifts you with your very own twin. Don't let the word “mimic” scare you off. This isn't a summon that's going to chomp on your face unexpectedly. Nope, this spirit ash calls forth a spectral version of your character. Try double teaming ahead!
One of the neat parts about the Mimic tear summon is that, unlike most other spirit ashes, it won't cost you Focus Points to use. Instead, it takes a chunk of your health to fuel your ghostly body double. When summoned, your mimic will have all the same armor, weapons, and spells as your Tarnished. Your body double can also use its own flasks and some other items, making them more likely to stay alive than many other spirits.
Mimic Tear spirit ash location: Nokron, Eternal City
To get to the Mimic Tear ashes, you'll want to have the Night's Sacred Ground Site of Grace unlocked. If you don't already, you're in for a bit of a journey, so buckle up.
The short version is that you'll need to begin the Ranni quest, fight one of the trickier Elden Ring bosses: Starscourge Radahn, and then head into Nokron, Eternal City by venturing deep into the earth near For Haight West. Assuming you've done all that, carry on your merry way through Nokron.
Appropriately, you'll have to fight a clone of your character as a boss to progress through this area. After defeating your twin, continue making your way clockwise and downwards along the rooftops of Nokron until you reach the Ancestral Woods site of grace. You're close now, and you'll need to do some additional parkour to reach the final destination.
Head southwest and downwards by way of more rooftops, ledges, and balconies. After you find a rooftop courtyard, head east across another ledge, a broken arch, and then through a window into a chapel. You've made it! You did bring a Stonesword Key, right? Unlock the fog wall in this chapel by using a key on the Imp Statue nearby and kill the last, unaware enemy standing between you and the chest containing those spirit ashes.
This walkthrough below by Gaming With Abyss does a great job of showing the speedy path through Nokron to the chapel.