This is the alternate ending for the game. To unlock this, you'd first need to complete the requirements to defeat the Allcat:
Upon defeating the Allcat, you'll receive the Clan Insignia. Equip this item (5th slot on your item slots), and complete the final boss fight at the Spelltower. Now watch (or skip) through the end game cutscenes until the part when the two characters are looking beyond the horizon. Cats will start joining them and this is when the achievement should unlock.
As usual, you can skip the battles with the "Instant Victory" option to quicken the process.