If you explore the Sky Islands a lot in The Legend of Zelda: Tears of the Kingdom, chances are you have come across chests with large stone slabs behind them. You may wonder what makes these chests so much more special than regular chests. However, inside you will find a Sage's Will. This is a remnant from the ancient Sages.
Sage's Wills can be used in a similar way to Light Blessings. They can then help Link strengthen his bond with one of the five Sages. In turn, that Sage will become more powerful. These Sage's Wills are extremely important to find. They can truly help Link gain the edge in battle with his Sages.
Updated by Erin Rice on July 5, 2023: The longer Tears of the Kingdom is out, the more information players will have on the game. This update includes information on how to obtain the Sages as well as information on how their upgrades help the Sages.
What Are Sage's Wills
There are 20 of these items scattered throughout the Sky Islands. Their description reads "The remnants of a former Sage's resolve. If you gather four of them and offer them to a goddess statue, you can deepen your connection with the Sages." Some of these are in the open, waiting for someone to open the chest containing them. Others require you to fight difficult bosses in the Sky Islands in order to get them. This means that while there are enough to upgrade each of the five Sages, you want to prioritize which Sages you want to upgrade first. According to the game, upgrading a Sage will increase the amount of damage they do by x1.8.
Considering you may not be able to take on some of the enemies required to obtain all 20, prioritization is key. Make sure you decide which Sages you use the most.
How To Use Sage's Wills
As mentioned before, these are similar to the Light Blessings. Once you have obtained 4, you can go pray at any Goddess Statue. The statue will ask if you want to use the Sage's Wills or Light Blessings. Once you select Sage's Will, she will ask which Sage you want to upgrade first. This is where you want to decide which Sages you use the most. That will tell you which to upgrade first. If you don't know, you can select any of them. It doesn't matter which order you upgrade them in. Here is a list of the Sages.
All Sage's Will Locations
This map shows the location of each of the 20 Sage's Wills. As you can see, they are scattered pretty evenly around the map. Here are all 20 going in a counter-clockwise fashion starting in the upper left of the map (the Hebra area).
West Hebra Sky Archipelago
You can find two different Sage's Wills in the West Hebra Sky Archipelago. For both, the Rospro Pass Skyview Tower is the best place to launch into the sky in order to land on these islands and grab the Sage's Wills.
This first one can be seen here. To reach this Sage's Will, you will need to travel to the bottom island and use the hooks laying around to create a railcar that will take you up to the island the Sage's Will is on. You will need to attach fans to it in order to help go to the island.
This island is home to a King Gleeok. It is one of the many difficult bosses you will need to face in order to obtain all the Sage's Wills. You must beat it in order to get this one. If you cannot kill a regular Gleeok yet, don't worry about this one just yet. This is a very difficult battle.
North Tabantha Sky Archipelago
You could potentially reach this area by one of the Skyview Towers, however, there is a waterfall near Rito Village that will allow you to reach the first island in this chain of islands. Once there, you will need to use the Zonai Devices available to you to reach the last island where the Sage's Will is located. You can use minecarts with Zonai Rockets and then just glide the rest of the way.
Tabantha Sky Archipelago
This Sage's Will is on an island known as Starview Island. The island contains a puzzle that allows you to unlock the Mayasiar Shrine on the island. However, you can also use the puzzle to unlock the ruins in the area so that you can grab this Sage's Will.
West Hyrule Sky Archipelago
This island is home to a Flux Construct III. With these bosses, the chest containing the Sage's Will is located on their shoulders. You can easily detach it and loot the chest or kill the boss.
East Gerudo Sky Archipelago
There are two Sage's Wills in this section of the sky.
This island contains another King Gleeok fight. If you aren't prepared for a battle, don't visit it.
This Sage's Will is on an island that has three levels. Using a Skyview Tower, you will land on the top level. You'll want to glide down to the middle level and grab a mirror (unless you already have one in your Zonai Devices). Drop the mirror down to the bottom level and then use the mirror to reflect sunlight onto the light on the bottom level. This will grant you access to the chest.
South Hyrule Sky Archipelago
This island has two ponds, one up top and one down low. You want to go to the bottom pond. There, you will notice that you can use your Ultrahand Ability to raise barriers to drain the pond. Do this and hold them open for 20 seconds or so. Then cast the Recall Ability on them, so you can run to the pond and grab the Sage's Will from the chest.
Faron Sky Archipelago
This island is guarded by another Flux Construct III. Either kill it or use Ultrahand on the chest.
Thunderhead Isles
This island is accessible from the beginning, however, there is a giant storm cloud blocking the map. It is recommended that you wait until completing the "Secret of the Ring Ruins" quest. The armor set obtained from that quest is pretty great for the area. On this section of the island, there is a rock wall blocking you from getting into the building that this Sage's Will is in. You can either destroy it or Ascend from the bottom of the island. Either way, the chest is inside.
Dragonhead Island
This Sage's Will is located around the mouth of the dragon. You're able to jump from the head onto this area. You want to make sure you have the "Secret of the Ring Ruins" quest completed for this either. Like the one above, it is possible to do so before, but it will be extremely difficult.
South Necluda Sky Archipelago
This island is guarded by a Flux Construct III as well. Either kill it or take the chest.
Eventide Island Sky
This island cannot be reached without using Zonai Devices to craft a vehicle that can traverse the sky. It is home to another King Gleeok. You want to wait on this one until you're ready for such a powerful enemy.
North Necluda Sky Archipelago
This island is the one completely covered in Luminous Stones. There is a small area to the north of the island that contains the chest.
Lanayru Sky Archipelago
There is one located here in the middle of the region. There is another one located very close to the Water Temple.
This Sage's Will is guarded by a Flux Construct III. Kill it or grab the chest and leave.
This island is actually a rotating sphere. You will want to go inside and move the crank around until there is only half an opening. From there, you will glide down and turn directly around to see another opening at the bottom of the sphere. The Sage's Will is inside.
Wellspring Island (The Water Temple)
While it is possible to get here before, your best option is to come to this island either during or after defeating the Water Temple. There is a Flux Construct II boss here. It is done the same way as the Flux Construct III bosses. Either kill it or grab the chest and run.
South Eldin Sky Archipelago
This is located on several connecting islands. You will want to take the minecarts up to the island shown on the map. Then you need to Ascend from where you are. This will bring you to an area with vines blocking your path. Destroy them and behind them is the Sage's Will.
Sokkala Sky Archipelago
This is another sphere. The chest is located at the bottom, just like last time. You will want to move the ball so that there is only half an opening just like before. From there, glide to the bottom part of the sphere. Inside is the chest.
Eldin Sky Archipelago
This final island is as difficult to get to as the one above Eventide Island. Thankfully, you can use Zonai fused vehicles from the top of Death Mountain to reach this island. There is a regular Zonai Construct on this small island. You can kill it or leave it alone. The Sage's Will is located inside the ruins on the island.