Read about the other medals:
The Dragonslayer's Crux
This medal is awarded to those who have defeated five great dragons, each from different dragonflights, in the name of the Light. This medal is made up of five dragon scales, each unique to the individual medal awarded. The fifth and central scale must be either from the chromatic or twilight dragonflight.
A cross is carved into each scale to sancify potentially dark powers of a dragon. The five dragon scale cross is pinned to the tabard with two blood red dragon teeth.
Dragons, thinking themselves demi-gods, sin against the Light with their arrogance. Only the one true Light holds dominion over the world. We must defeat these creatures to remind them that they are nothing more than glorified animals. Their heresy must be stopped.
(( To be clear: defining "great" - it is intended that these dragons are raid-level or world-boss dragons, that these should be done on your CT character, at-level, or whilst Timewalking, and you should have screenshots, or have an officer with you or online and aware whilst doing it.))