Owning this DLC unlocks the premium track of the battle pass. It covers the current season and every season after it. You buy it once. It does not renew.
What it gives: two perks from the moment you own it. You can sell on the Steam Community Market. That covers your tradeable pets and gear. It also covers your Abyss Cores. The forge gets auto-fusion. It fuses your spare items in the rarities you tick.
Each season's tiers hold forge seals and forge stones. There are also World Boss stones and runes. The rest is gold, essence and shards.
A forge seal keeps an item from breaking when an upgrade fails. More seals and stones mean stronger gear.
What it does not give: no chests, no pets. It gives no extra pass XP. Tiers fill at the same pace with or without the DLC.
How it works: pass XP comes from playing. Beating the boss of a new stage gives XP. Opening chests by hand gives XP too. So do the arena, duels and the World Boss. At each tier you claim both rewards, free and premium. Each new season the pass starts over. It stays premium for you.
Achievements are not affected: every achievement in Idle Legions can be earned without this DLC.