I opened 10 packs and there was no Legendary card
Whenever a new expansion set is released, you are guaranteed to receive a Legendary card within your first 10 packs opened for that set, regardless of how the packs were obtained. Once you've received your first guaranteed Legendary, whether it came from your first, fifth, or tenth pack, further Legendary cards are subject to their normal rarity.
Additionally, any Legendary cards received from a pack will be a Legendary card from the same set that you don't already own. This applies to the Classic packs as well as expansion sets. Once you own either the golden or non-golden version of a Legendary card, you won't get a duplicate of it from packs until you own all the Legendary cards from that set.