Hero events are the best way to obtain a legendary hero.
If you are a free-to-play gamer, it is recommended that you first obtain a legendary hero and keep upgrading it to level 15 before investing in a different hero. Currently the most recommended heroes are Chione and Apollo.
Current event
Dec 8 ~ Dec 14
Mephisto
Past events
Dec 1 ~ Dec 7
Dracula, Poseidon, Wukong, Harrison, Beowulf, Drake
Nov 24 ~ Nov 30
Aly
Nov 17 ~ Nov 23
Apollo, Davison, Dracula, Drake, Nezha, Poseidon
Nov 10 ~ Nov 16
Apollo
Nov 3 ~ Nov 9
Davison, Chione, Beowulf, Wukong
Oct 27 ~ Nov 2
Dracula
Oct 20 ~ Oct 26
Chione, Nezha, Harrison, Horus
Oct 13 ~ Oct 19
Athena
Oct 10 ~ Oct 12
Mephisto, Poseidon, Drake, Wukong
Sept 29 ~ Oct 5
Davison
Sept 22 ~ Sept 28
Chione, Horus, Dracula, Mephisto
Sept 15 ~ Sept 21
Beowulf
Sept 8 ~ Sept 14
Chione, Poseidon, Drake, Nezha
Sept 1 ~ Sept 7
Davison
Aug 25 ~ Aug 31
Chione, Poseidon, Drake, Nezha
Aug 18 ~ Aug 24
Horus
Aug 11 ~ Aug 17
Nezha, Chione, Poseidon, Mephisto
Aug 4 ~ Aug 10
Davison
July 28 ~ Aug 3
Poseidon, Apollo, Wukong, Chione
July 21 ~ July 27
Horus
06/16 ~ 06/22
Wukong, Apollo, Hohenheim, Drake
June 9 ~ June 15
Davison
June 2 ~ June 8
Poseidon, Nezha, Harrison, Wukong
May 26 ~ June 1
Poseidon
May 12 ~ May 18
Apollo, Poseidon, Dracula, Drake
May 12 ~ May 18
Davison
April 28 ~ May 4
Apollo, Poseidon, Wukong, Drake
April 14 ~ April 20
Selene, Dracula, Davison, Apollo
Mar. 17 ~ Mar. 23
Wukong, Davison, Apollo, ???
Mar. 10 ~ Mar. 15
Dracula
Mar. 3 - Mar. 8
Davison
Feb 26 - Mar. 3
Drake, Harrison, Apollo, ???
Feb 8 ~ Feb 14
Davison
Jan 20 ~ Jan 23
Apollo
Jan 1 ~ Jan 10
Poseidon