Event Legendary heroes can't be bought with shards or rolled with gems. Cards for these are given as rewards for completing occasional events. They can also be rolled with Opals.
“Graveshadow is exactly the same as CC's Moltanica. Graveshadow can't be rolled, but is listed as...Graveshadow
“Nirgali in Clash of Desert is largely the same as Vlad Dracula from Castle Clash with the...Nirgali
“Maahes is exactly the same as CC's Aries except that he is only available from special events...Maahes