Scalerein

A mithral chain with a strangely-curved grappling hook at one end and a thick manacle at the other, specifically designed to engage with dragons.

The chain has a base AC of 15 and 30 HP. If the chain is broken, the artifact automatically casts feather fall as a reaction. You can use a bonus action to open and close the manacle at the end of Scalerein by manipulating a small lever on its side. As an action, you can make a ranged weapon attack with Scalerein. If the target is a dragon, the hook lodges itself between the creature's scales on a hit.

If the grappling hook is lodged in the scales of a dragon which is at least one size category larger than you, you can use an action to to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target’s Dexterity (Acrobatics) check. If you win the contest, you successfully pull yourself onto the target's body and enter its space. While in the target’s space, you move with the target and have advantage on attack rolls against it.

You can move around within the target's space, treating the space as difficult terrain. The target’s ability to attack you depends on the your location, and is left to the DM's discretion. The dragon can dislodge you creature as an action—knocking you off, scraping you against a wall, or grabbing and throwing you—by making a Strength (Athletics)  check contested by your Strength (Athletics)  or Dexterity (Acrobatics) check. You choose which ability to use.

If the grappling hook is lodged in the scales of a dragon which is the same size or smaller than you, you can use an action to to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target’s Dexterity (Acrobatics) check. If you win the contest, the target is grappled.