ゼルダの伝説：神々のトライフォース
The Legend of Zelda: Triforce of the Gods

As Link enters the clearing where the Master Sword rests, forest creatures hop across the screen. In any event, he can now use the Writings of Mudora to read the inscription on the pedestal:

That's pretty much what everyone else said. Link stands behind the pedestal and places his hands on the hilt of the sword... the crests float around him, glowing... the sword shines mightily... dramatic music plays... and in a burst of light that clears away the mist, Link pulls out the sword and holds it aloft!

In a Randomizer seed, anything that isn't required to get the three crests (pendants) can be here. More often than not, it's something disappointing, especially given that you still need all three crests to collect it. Reading the inscription at least gives an indication of what's on the pedestal.

The bluish sword glints brightly in the sunlight. Suddenly...

Not only does the Master Sword look more impressive than Link's old sword, it also makes a deeper sound when swung, has slightly better reach, inflicts twice as much damage (usually), and as long as Link is at full health additionally launches damaging energy bolts (the same power as the original sword) on attack.

It's also the bare minimum required to inflict any damage on the final boss, and required to get the items from the tablets. Speaking of which, you can climb back up the mountain and get Ether now, but it's rather out of the way and not, I think, worth the trouble just yet, so we'll save that for later.

As Link leaves the clearing...

Link rushes to the church, but he's too late. Princess Zelda is gone, and the abbot lies motionless on the ground.

Having uttered his final words the poor fellow disappears. Odd, that. Regardless, coming here is optional if you know where to go already.

Well, it's time to go to the castle to save Princess Zelda, again.

Security is tighter, but Link is stronger, and fights past the guards with little difficulty. A single swing of the Master Sword eradicates the tower barrier that had previously seemed unbreakable.

I could include another version of the map here to show that, but nah. Nothing else would be any different, and you can barely tell the castle tower was grayed out to begin with. Incidentally, you can also pass the barrier using the Magic Manteau in a Randomizer run, if that comes up before the Master Sword does.

The tower is crawling with stronger monsters and guards than the main castle, some of them left-handed to make them more difficult to fight, and many of the rooms along the way are dark on top of everything else. Partway up, Link finds another telepathic tile:

Link finally reaches the top of the tower. Princess Zelda lies on the sacrificial altar, with Agahnim looming over her.

殿 (dono) is an archaic and somewhat respectful name suffix, while ku ku ku is a sinister chuckle.

He does some evil magicky thing and Zelda vanishes.

He teleports out of the room through the north wall. Link slashes at the curtains to find an opening, and follows to the tower roof.

In a Randomizer run, you still need a sword to pass the curtain (except on the Swordless setting), so the Magic Manteau alone isn't enough to face Agahnim.

This is arguably the first boss where strategy is critical. Agahnim teleports around the room, and has three attacks. One is a lightning spread that randomly covers most of the screen, but never hits the top corners, so is easy to avoid by standing there. One is a homing circle of energy balls that splits into small, non-homing, and blockable (though Link has to get his shield back up in time) balls when hit with the Master Sword. The most important one is a large energy ball that the Master Sword knocks away entirely. With a bit of aiming, Link can hit these balls back at Agahnim, inflicting damage. This is the only way to hurt him, since, hitting Agahnim directly is not only as ineffective as Sahasralah warned, but actually hurts Link.

Oddly, the bug-catching net affects Agahnim's attacks the same way as the Master Sword, though it's harder to aim with. The Hammer and even the starting sword also work if you happen to have either one, not that it makes any difference in a regular playthrough.

Six hits later, Agahnim has enough of getting beat up.

Agahnim begins spinning around rapidly, and disappears as the screen distorts as it did at the warp tile on Mount Hebra...

For Randomizer purposes, Hyrule Castle Tower contains only two chests, as well as two enemies that drop keys (which will never be randomized and always appear in their usual locations). Since there are also four locked doors in the tower, this means that these two chests will always contain keys, unless keys are shuffled beyond their usual dungeons. In other words, under typical settings, the only reason to come here is to beat Agahnim, and that's usually optional.

To the best of my understanding, beating Agahnim is logically required in a Randomizer seed only if one of the following is true: 1) The victory condition is All Dungeons. 2) There's something you need on "Lumberjack Ledge", the location beyond the tree the lumberjacks are cutting down (a piece of heart is there in a regular playthrough), since beating Agahnim is what results in them leaving the tree so you can dash into it (you can check what this item is in advance by looking in the cave behind their house). 3) The Moon Pearl (or something that gives access to it) is on the pyramid. 4) You have no Dark World access (not counting the first warp tile on Hebra Mountain) without beating Agahnim; that is, you lack and cannot reach one or both of the Hammer or the Power Gloves. 5) You have western Dark World access via the Powerful Gloves (Titan's Mitts), but no Hammer or Flippers, and need to reach something on the eastern side, most often but not necessarily the Temple of Darkness. 6) The Pyramid Fairy has something you need, but you don't have the Hammer to get the jumbo bomb across the bridge and to the pyramid.

If willing and able to use glitches, though, there are at least two ways to bypass case 5 that are allowed in competitive Randomizer runs. The Qirn Jump, a tricky Fake Flippers setup that involves a precisely-timed jump into the river north of the broken bridge after tossing a Bomb so that the blast will push you south into the screen transition, lets you swim across the river without Flippers. With the Pegasus Shoes, it's also possible to set up a Water Walk by dashing along the edge of one of the pitfalls in Skull Woods and bonking into something to arm the glitch, then heading over to the river and jumping in from the north.

Link appears on top of a pyramid. In the distance to the north is a mountain similar to Hebra Mountain but more sinister-looking, topped by a tower silhouetted in a foreboding sun. The sky is an unhealthy mix of maroon and brown, adding to the foul feel of the place. Link hears a voice in his head again:

As Sahasralah suggests by his talk of a channel between world, walking through the Hyrule Castle gate will now transport Link to the Dark World.

Link does his usual victory spin, then sets about exploring this strange Dark World...