Of the 100 existing champions in Lore & Legends, 40 of them need to be "unlocked" before they can be played. Each one has its own unique unlock condition, creating mini objectives that players can try to complete.
Many of these follow naturally when building towards a trait, but there are also others with far more specific requirements.

For example, one unlock may ask you to collect X star levels of a single trait. Another could lean towards certain items. The difficulty varies greatly, but the tougher the challenge, the greater the reward.
They've even introduced a handful of 7-Cost Champions, all of which must be unlocked. For the most part, these function the same as a 5-Cost, but are ranked up to showcase their strength and highlight your achievement.
Note: I won't spoil any of them here, but if you're interested, check out our Unlockable Champions page to see every unlock condition.

Once you complete a champion's unlock condition, you are guaranteed to see it in your next shop (furthest from the right). This is great as it allows you to plan ahead and avoid relying solely on RNG.
These champions still follow the usual bag sizes, meaning they can be contested by anyone else who unlocks them. As you'd expect, selling them also puts them back into the pool.
Overall, it's a very clever system that adds controlled complexity to the game. With this, they can have a wide array of champs without intimidating the players or diluting the set. Well done Riot.