Elite rune dragons appear occasionally while the player is killing regular rune dragons. When one appears, the player is notified with a chat message. The dragon will fight the player only, unless three minutes have passed without the player attacking it, or if they teleport out of the area. When either of the conditions are met, any player can attack the elite rune dragon.
Elite rune dragons share the same mechanics as their normal variants, but boast higher combat stats. Care should be taken in longer encounters during its enrage phase, as they can reach the point of easily killing a player in one hit.
The Kethsi outfit scrolls are only dropped by elite rune dragons. Slayer tasks have no effect on elite rune dragons' drops; thus it is possible to obtain the boot upgrades or Kethsi scroll whilst off-task. It is a guaranteed drop from "Barry", a unique elite rune dragon that has a 1 in 512 chance of spawning.
Starts with 69,424 life points. Any attacks during the transition will be reduced to 10% damage. 
The dragon begins flying and becomes immune to melee attacks. Dragonbane is no longer required to deal full damage. In addition, the dragon stops using melee attacks and replaces its standard dragonfire attack with the enhanced version.
Throughout this phase, the rune dragon may breathe a bright blue plume of enhanced dragonfire, dealing magic damage. This attack cannot be fully blocked by a super anti-fire or wyrmfire potion, and will deal a minimum of 450 magic damage. However, if the player is far enough from the dragon, a small cloud appears under the player indicating the enhanced dragonfire is being used and can be avoided by moving away from it.
^ abJagex. Mod Stu's Twitter account. 11 January 2016. Mod Stu: "If today's update goes as planned, Kethsi outfit scroll dropchance will be 1/2048 -> 1/200, and a guaranteed drop from Barry the Destroyer."
^ abJagex. Patch Notes | 22/08. RuneScape Official Forums. 22 August 2016. "Made changes to rune dragons' phase transitions to allow players to continue attacking throughout. Damage that overshoots the lifepoint thresholds will no longer be healed back, but that hit, and hits during transition, are reduced to 10% power."*