Collecting all 12 Zodiac Relics in Kingdom Hearts HD 2.8 will unlock a boss rush fight from the main room in World Within. Defeating them all will give you a lot of EXP, so it's worth collecting each one. However, the treasure chests containing them will only appear once you've beaten the game already. Run a New Game+ after doing so to start collecting them.
Kingdom Hearts HD 2.8: Castle Town
Aries: It can be found pass the Town Square. Look for a building with four boxes beside its right wall. The chest with the Aries relic is beside it.
Aquarius: From the Town Square, head to the left and look for a ruined house. Go to the cog and jump to where the debris are. The chest with the Aquarius relic is on there.
Virgo: It can be found left of the Town Square, but go through the path leading to the last cog. Once you've passed the archway, look to the left and you'll find the chest containing the Virgo relic.
Sagittarius: On the same path, past the archway, climb up using the debris to get to the highest point of the map. At the end, there's a chest with the Sagittarius relic.
Kingdom Hearts HD 2.8: The World Within
Cancer: Find the symbol of the Sagittarius (a man with a bow and arrow) at the ground in the main room. Go past it and turn left (do not climb the stairs). The Cancer relic is inside the chest there.
Pisces: Look for the mirror in front of the Scorpio and Libra symbols. It should warp you to a room with a long winding staircase. Enter the mirror and look behind you to find the chest with the Pisces relic.
Gemini: Find the mirror that takes you to a room with pillars and a mirrored floor. Slash the glowing pillar to your right. Then, find the second glowing pillar and slash it again. Find the third glowing pillar and you'll find the chest near it.
Leo: In the main hall, warp through the mirror that takes you to the room with the mine. Interact with the first mirror to spawn a platform. Follow the path and you'll find a second mirror, beside which is the chest with the Leo relic.
Libra: After getting the relic, make your way to the large area, where some Heartless will be waiting for you. You can find the chest with the Libra relic here.
Kingdom Hearts HD 2.8: Forest Of Thorns
Taurus: Past the save point, jump down into a dip in the terrain. The chest containing the Taurus relic is right next to the large chest with the map.
Scorpio: Instead of jumping down, grind the rail to head to the next area. Go straight until you reach a small clearing. The chest will be immediately to your right.
Kingdom Hearts HD 2.8: Depths Of Darkness
Capricorn: When you come across a large empty area with a bright light, look to your right for a pillar. The Capricorn relic is in the chest on top of the said pillar.
How To Get To The Secret Boss Rush Fight
When you have collected all 12 Zodiac relics in Kingdom Hearts HD 2.8 (Kingdom Hearts 0.2), go back to the main hall in The World Within. In the middle of the room, there should be a new mirror. Interact with it and it will take you to a battle with 12 bosses. Each one is not as easy as any boss you've fought with before, so be extra careful.