Complex sentence: Due to delays from the Marvel art department, whose cavalier and lackadaisical approach to the project brought it to a screeching halt, the booster packs did not hit store shelves for months after the release of the starter decks.
Split into simpler sentences: Due to delays from the Marvel art department, whose approach to the project could be best described as 'relaxed', brought the project to a screeching halt. The outcome being that the booster packs did not hit store shelves for months after the release of the starter decks.