Because you are buying a season pass, not a single DLC, its three. The Season pass gets you Wild-card, the DLC with Beastbrood Tieflings (Rakshasa), Shackleborn (Kyton), Faultspawn Tieflings (Asura), Motherless Tieflings (Qlippoth), Pitborn Tiefling (Abyssal), Demodand Tieflings (Demodands), Hellspawn Tieflings (Devil), Div Tieflings (Divs), Deamon Tieflings (Daemons), regular Tieflings, Twin Tiefling Companions and their Companion Quests, possiblely some Tiefling portraits, the Kinetist Base Class with Fire/Water/Earth/Air elemets and the Dark Elementalist, Pyscokinestist, and Kinetic Knight Archetypes.
In 2019 you will get next DLC which is a second story that effects what happens in the main story some how.
The last of the Season pass DLCs in 2019 as well is a massive dungeon that can be explored in the main story or as part of a seperate dungeon crawl game.