Malice: Starts as a Legendary ancient and was won in the first kingdom event. While she is not currently obtainable Space Ape has said she will eventually be found in chests. Event ancients get an exclusivity period and then slowly enter regular play like Yagumo. We list her here to make comparison easier.
Viscaria and Talasin: Ascended by use of the Ancient Dragon Heart. In order to get a Heart you can complete the Trial of Dragon. This requires you to have all 5 dragons, raise them all to max level, and get 450 wins on each. It was also available for being in a top Kingdom during the Dragon Heart Event.
Foreveil: Ascended by use of White Eagle Kris to get from Superior to Rare and Rare to Epic. White Eagle Kris could be won from Arena Seasons(Dec ’15), or White Eagle Kris Chests. Ascended from Epic to Legendary by use of the Ancient Dragon Heart.
Emberclaw: Ascended by use of Scale of Elahel which was obtained from Infernal Hoard Chests to get from Rare to Epic. Ascended from Epic to Legendary by use of the Ancient Dragon Heart.
Since most people will be interested in dragon ascension for the dragon itself we are combining all 5 legendary dragons into one page. If you are interested in Foreveil or Talasin as a Legendary Ancients you can see their third spells too.