The Queen's Domain Holy Sword Endu is the traversal weapon of the game and it never leaves your loadout. It is taken from the pedestal in the Endu Shrine after you beat the Shrine Guardian, and its Wind Dash is the only way across gaps for the rest of the game.
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Where the Queen's Domain Holy Sword Endu comes from
The Shrine Guardian is a heavily armored skeleton with a big cleaver. Like every other skeleton it is weak to Impact, so the Wooden Club from the crypt beats the Rusted Short Sword. Its overhead swing reaches much further than it looks, so step well back when the blade goes up and punish the long recovery with a charged hit.
There is no Wind Dash yet, so positioning is everything, and the rocks in the arena catch you if you retreat past them. Beating the Guardian gives you three things at once: the Holy Sword Endu, the first Wind Gem for one mid-air Wind Dash, and the Stone Duck Figurine that opens Umbada's Trading Outpost from any Crystal Monument.
The opening walkthrough has the shrine fight step by step.
What each Wind Gem adds
There are five Wind Gems in the island, and each one adds a single diamond to the Wind Dash meter, which is one more mid-air dash. Collecting all five unlocks the Feel the Wind achievement.
The first three arrive early. One comes from the Shrine Guardian, one from the rocks between the pillars at the second save point, and one inside Obmuer Temple past the second Trident key. The remaining two are elsewhere on the island and are worth collecting whenever you pass near a hidden route.
The Storm Gem is a different item and it is not a dash upgrade. If a gap is still too wide after a dash, the answer is another Wind Gem, never the Storm Gem.
The sword jump guide shows how to chain the dashes the gems give you.
Wind Dash trails in a dungeon hall: every Wind Gem adds one more of these mid-air dashes.
The Storm Gem turns on the special attack
The Storm Gem, recovered in the Tower of Knowledge, switches on Endu's special attack meter. The meter fills as you kill enemies, and once it is full the attack fires with R on keyboard or Triangle/Y on controller. You do not need to be holding the sword to fire it.
The Storm Gem guide covers the upgrade itself.
The three steps to True Holy Sword Akunra
Level the Holy Sword Endu to 10 so that it becomes Soul Bound Endu.
Level Soul Bound Endu to 10 as well.
With all five Wind Gems and the Storm Gem in hand, take the sword to the Altar of Purification behind the large door in the Verdant Woods area of Verdant Hollow. That door is the one the eight Sipeha seals open, and it opens once every green magic object there is dispelled with the Mask of Second Sight.
The result is the True Holy Sword Akunra, which dashes ten times and carries the strongest charged swing in the game. Akunra is also the weapon tied to the second ending, and it opens a back way into the memento altar used for the third ending, so it is worth chasing even if you are not after those.
The Petrified Garden seals guide has all eight seal locations.
Why the sword is worth the effort
Endu is the one weapon you never swap out, because its Wind Dash is how you reach every new area, every chest on a high ledge and every secret room on the far side of a gap. Akunra adds ten dashes on top of that, which is enough reach to open routes that stay closed otherwise.