The free DLC comes out today and includes the "Mario: Oh, Brothers" and "Super Sonic Sergios" quests. Completing the quests unlocks materials for the Mario and Sonic costumes respectively. Players can also craft the Invincible Hammer, a Mario-themed weapon.
The DLC includes plenty of other content, ranging from additional quests to new guild card backgrounds. So if you find the Mario and Sonic costumes positively creepy like I do, at least there's other content to keep you busy.
Then again, this is a prime opportunity for Link, Mario, and Sonic roleplaying sessions. Get to work people!