The 
Cinderous Soul Rune is a glowing altar buried deep inside the Lost Citadel, and it exists for one purpose. It is the anchor that summons Echo of Tyros, First Flamebearer, the fire boss of Iron’s Spells and Spellbooks. You will not stumble into this fight by accident, because the boss only appears when you call it to the rune.
The Citadel’s old occupants bound the First Flamebearer’s soul to this stone through a ritual of fire and souls. Their burned research books, scattered in the same halls, describe collecting loose souls and using them as fuel. The rune is what remains of that work, waiting for someone to ring it back to life.
You cannot craft this block or carry it home, so the fight always happens where the Citadel placed the rune. Treat the room around it as your arena and prepare fully before you trigger anything.
Obtaining
You do not make the Cinderous Soul Rune. It generates as a fixed fixture inside the Lost Citadel, a structure found in the Nether, set into the core of the structure near the boss altar. Look for the burned ritual books in the same area, since they spell out what the rune is for.
You also cannot take it with you. The block drops nothing when broken, resists 
TNT completely, and 
Pistons cannot push it, so the altar stays exactly where it spawned. There is no reason to mine it in Survival.
Only Creative mode lets you place your own copy. Use the command below to give yourself the block item or set one directly into the world.
Usage
The rune has a single job. It lets a 
Cinderous Soulcaller summon Echo of Tyros. Stand near the altar, use the Soulcaller, and the boss rises from the stone already facing you.
Find the Rune
Inside the Lost Citadel
Locate the Lost Citadel and work your way to the altar room where the rune glows. This is also where the boss will spawn, so clear the space and note your escape routes before you go any further. You can use the locate command below, it will only work in the Nether, as that’s where the Lost Citadel spawns.
Craft a Cinderous Soulcaller
The key that rings the altar
The Cinderous Soulcaller is a separate item you build at a 
Crafting Table. It takes six 
Cinder Essence, two 
Netherite Scrap, and one 
Echo Shard, shown in the recipe below. Cinder Essence drops from Ancient Knights, so farm a few before you make the trip.
Ring the Soulcaller
Right-click near the rune
Stand within about 22 blocks of the rune, then right-click while holding the Soulcaller. It consumes one Soulcaller and Echo of Tyros appears on the altar at once. Nothing happens if you are too far away or too far below the stone.
Only one Echo of Tyros can exist near the rune at a time. If one is already alive, the Soulcaller answers with “An echo has already answered” and is not used up. The Soulcaller also has a short cooldown of about four seconds between rings, so you cannot spam summons.
Bring a 
Fire Resistance potion before you ring the Soulcaller, since Echo of Tyros is a fire boss and the fight opens the instant it appears. Setting a spawn point near the rune, such as with a 
Respawn Anchor, also saves the walk back if you go down before the fight is over.
Echo of Tyros appears the moment you ring the Soulcaller, and the fight begins right at the altar with no windup. Do not use it until your armor, healing, and buffs are ready. See the Echo of Tyros, First Flamebearer guide for the boss fight itself.
Mechanics
The Cinderous Soul Rune is built to be permanent. It has a hardness of 40, which makes it slow to break by hand, and a blast resistance of 1200, the same as 
Obsidian, so explosions cannot clear it. Pistons cannot push it either.
The block also gives off full light at level 15, so the altar stays lit like a beacon in the dark Citadel. Since it never drops an item, you cannot relocate it to a safer room. Build your defenses and staging around the rune instead of trying to move the fight.
FAQ
Craft a Cinderous Soulcaller from six Cinder Essence, two Netherite Scrap, and one Echo Shard. Stand within about 22 blocks of the rune and no more than about two blocks below it, then right-click while holding the Soulcaller. The boss spawns on the altar and the item is consumed.
No. There is no crafting recipe, and the block drops nothing when broken. It only appears inside the Lost Citadel, or you can place one yourself in Creative mode using commands.
That message means no rune is in range, or you are standing more than about two blocks below one. Move within about 22 blocks of a Cinderous Soul Rune and stand no more than about two blocks below it. If you see “An echo has already answered” instead, an Echo of Tyros is already alive nearby.
It generates inside the Lost Citadel, a structure found in the Nether, set into the boss altar at the heart of the structure. Burned ritual books nearby describe the soul-and-fire process that bound Echo of Tyros to the rune.
No. The rune drops nothing when mined, shrugs off explosions with 1200 blast resistance, and pistons cannot push it. The fight always happens where the Citadel placed the altar, so build your defenses around that spot.
