Cedar! :)

Spotted the Hero in the Depths today. I attempted to approach while disguised as a member of the Zonai Survey Team.

Bad idea. I was attacked immediately and viciously without hesitation. I barely escaped with my life!

That... was not the Hero.

Was that the Hero's Shadow spoken of in the old legends? It is said the Shadow would mirror the Hero's appearance and fighting skills... I believe it after that encounter.

ATTENTION YIGA CLAN MEMBERS: If you spot this figure during your patrols, DO NOT APPROACH. This 'Shadow' will viciously attack anything and anyone that dares get near, even the Yiga Clan.

Beware the Hero's Shadow.

Glory to Master Kohga.

• Dark Link would be patrolling the Depths, and upon spotting you, he will chase you down and attack, similar to the Gloom Hands. But unlike the Gloom Hands, you cannot escape by climbing to high ground and waiting for him to vanish. He will simply teleport up to you.
• You can either deal enough damage to him, which will make him leave and reappear elsewhere in the Depths (like how the Yiga flee when defeated), or you can teleport far enough away so he loses track of you. Thankfully, he will not follow you to the Surface.
• If he spots you while you're wearing the Dark Link outfit, he will be temporarily confused, giving you very limited time to run away or hide before he figures it out and starts attacking you. Similar to the Lynel Mask.
• Also while you're wearing the Dark Link outfit, the Yiga will not ambush you, because they think you're the Hero's Shadow.
• Dark Link's number of hearts will match that of the player's, just like in OoT. The stronger you get, the stronger he gets too.
• He would also have the smartest enemy AI in the game, besides Ganondorf himself:
• He dodges any thrown items or arrows shot at him, which means he cannot be cheesed with bombs. He will simply hop out of the blast radius before it lands. You must fight him in close melee combat.
• He will mirror your strikes perfectly if you fight him with a one-handed sword and shield. However, he only wields a sword and shield, meaning you can attack him more easily with a different weapon, like... oh I don't know... a two-handed hammer? 😉

• He carries a leveled sword and shield, depending on your world level (similar to the way Silver bokos spawn instead of Reds). The first time you meet him, he's carrying a traveler's sword and shield. The second time, a feathered edge and kite shield, etc. All his swords are fused with Dark Shards, except for his final upgrade.
• He's at his most dangerous when you get the Master Sword. He carries a dark version of the Hylian shield with a Gloom Toll effect (you lose a single gloom heart whenever you block with it, unless you do a perfect parry), and a blackened Master Sword that looks suspiciously like Ghirahim's.
• Once he has the dark sword, he can shoot you out of trees/off of cliffs with a Master Sword beam.
• Wearing Majora's Mask alerts him to your location. You have to decide whether you want to face that stone-covered Lynel, or risk an encounter with Dark Link.
• Similar rumors in the Zonai Survey Team start surrounding Link, just like with Puppet Zelda's appearances.
• A final fight against him at the very bottom of the Hyrule Castle Chasm, before you go into the tunnel with the Like-Likes and Horriblins. If you run past him, he shows up as a boss to beat in the War in the Depths of Hyrule; maybe with dark Sages to even it out, a la Astor's fight in Korok Forest in AoC.
• Killing him doesn't actually grant you his sword, just something called a Dark Shard. Fusing it to a weapon gives it Gloom Toll, but also gives it the ability to shoot lasers like the Master Sword at any level.
• He probably also drops a few dark clumps.
• He has a haunting theme that he whistles along to as he patrols. It's a slowed down version of the original LoZ Game Over theme.
• Whenever the camera is pointed at him, he always looks in your direction, even if he isn't in range to see you. If he IS in range, though, watch out.
• His arm looks Zonai like Link's. Instead of giving him Ultrahand/Ascend/etc, however, it lets him dismantle any Zonai device you have with a wave of his hand. This catches people who are flying around the depths on hovercraft off-guard.
• He can be temporarily distracted by a Silent Princess or a Sundelion. He'll pick it up, examine it, then crush it, making a glowing particle effect scatter from his hand into the breeze.
• His teleport/death animation looks like he's breaking apart into poes that quickly scatter.
• His health is restored if he stands on Gloom. He doesn't actively seek it out, though.
• He can flurry rush your attacks just like Ganondorf can. He can't flurry rush a flurry rush, however.
• If his pathing comes across water, he'll stop to crouch down and stare at it for awhile before moving on.
• Light Dragon shards/talons/scales/etc dispel him instantly.
• There's only one of him around at a time. He has a set path he follows that almost perfectly mimics the Light Dragon. This path is only deviated from if you wear Majora's Mask, after which he teleports near your location and runs to whatever coordinates you last stood on flat land while wearing it are.
• If you have the Master Sword and are nearby, but hidden, his dark sword will make a Fi alert noise, then he'll go into search mode and look for you.
• When he dies while holding the dark sword, he'll drop the shield, but the sword will scatter into red light with a similar sound/effect to when the Master Sword loses its power.
• A red aurora follows him in the sky above. This prompts players into investigating, and serves as an early warning/way to locate once you know what he is. It's easy to miss if you aren't looking up.