Pity Timer: Legendary in the first 10 Packs only for the newest Expansion
Hearthstone has a cool system named Pity Timer. It guranteeds you get a epic cards after 10 packs and a Legendary after 40 packs.
The Game also provide something cool for the first few packs you open for a new Expansion:
Everyone will be guranteed to open a Legendary card within their first 10 packs of the new set.
– Hearthstone Update: June 22, 2017
In a ticket by the reddit user Nightwish001 published his situation:
The user bought 11 packs of Goblins vs Gnomes and expected to get a Legendary guranteed in the first 10 packs, but he didn’t.
The Blizzard Support made clear that this rule is only for the newest set, if you buy packs from older expansion you are not guranteed a Legendary in the first 10 packs, even if you never bought any.
The guranteed Legendary in the first 10 packs opened is something that happens whenever a new expansion is released. […] It is correct you didn’t get one in the packs of Goblins vs Gnomes as that is an old expansion.
– Blizzard Support (Source)