This is not the full Collection from Steam, but it was the closest to choose entry in the list.
The key is from Humble Indie Bundle #7 for Dungeon Defenders + All DLC (Which were available when this Bundle was on sale.) and includes following DLCs according to Humble Bundle support:

Dungeon Defenders Halloween Costume Pack
Dungeon Defenders Halloween Mission Pack
Dungeon Defenders: Penny Arcade Character Pack
Dungeon Defenders The Great Turkey Hunt! Mission & Costumes
Dungeon Defenders New Heroes DLC
Dungeon Defenders Warping Core Challenge Mission Pack
Dungeon Defenders - Etherian Holiday Extravaganza
Dungeon Defenders - Quest for the Lost Eternia Shards Part 1
Dungeon Defenders - Quest for the Lost Eternia Shards Part 2
Dungeon Defenders: Quest for the Lost Eternia Shards Part 3
Dungeon Defenders - Quest for the Lost Eternia Shards Part 4
Dungeon Defenders: Assault Mission Pack
Dungeon Defenders: Barbarian Hero DLC
Dungeon Defenders: Etherian Festival of Love
Dungeon Defenders - President's Day Surprise
Dungeon Defenders Lucky Costume Pack
Dungeon Defenders: Series EV Hero DLC
Dungeon Defenders - Karathiki Jungle Mission Pack
Dungeon Defenders: City in the Cliffs Mission Pack
Dungeon Defenders: Summoner Hero DLC
Dungeon Defenders: Talay Mining Complex Mission Pack
Dungeon Defenders: Jester Hero DLC
Dungeon Defenders Anniversary Pack