Sapphiron - "Slain by the Lich King" - Killed and immediately raised by Arthas (~technically pre-LK~). Better known for chillin' with Kel'Thuzad and eventually being put to rest by adventurers. Sort of twice. Sort of three times.
Arcanagos - "Slain by Medivh" - Really more like slain by Medivh*. Put to rest by adventurers. Twice.
Balacgos - "Slain by own creation" - He done goofed. The Bane goes on to play a role in other important events
Arygos - "Slain by Infinite Dragonflight" - Betrayed might be more accurate, but it's splitting hairs. Involved/shown in Tarecgosa's chain
Saragosa - "Slain by unknown adventurer" - Not a mystery at all. Not sure why they felt the need to include 'unknown' here.
Eldragosa - "Unknown" - We know she's dead because we talk to her ghost
Umbrelskul - "Unknown" - Should really be 'slain by adventurers' (little surprised they didn't have this one be updated)
Sindragosa- "Neltharion's Betrayal" - More well known for her being raised by Arthas and eventually put to rest by adventurers (and haunting all our loading screens)
Malygos - "Slain by Adventurers" - Dies during his own raid/encounter
Madrigosa - "Slain by Brutallus" - Death witnessed by adventurers, then put to rest.
Kaldrigos - "Slain by Dwarves" - Referenced via archeology and a 'Legiondary'
Tarecgosa - "Self Sacrifice" - Dies during this quest/her chain.
Dragons added/known as of this quest:
Queligosa
Omigos
Egrigos
Ashigos
Shiligos
Vetrigosa
Farkigos
Pilkigosa
Upligosa
Yigrigosa
Brigligos
Wulfigosa
Filigosa
Relixigos
Wenigosa
Curigosa
Duligosa
Xalkigos
Emigos
Stelagos
Grevigos
Xuligosa
Hexigosa
Meligos
Veligos
Mirugosa
Slepigos
(some of these do reference known/witnessed events in their causes of death)
(seems like kind of a lot of mention/foreshadowing of disease, but I might be grasping at straws)