Test (Mantis & Groot)
and are available from the starting 5/31 (PT).
A higher drop rate of Mantis and Groot (Sapling) Tsums for a limited time only (until 6/3 11:59PM PT)! Plus, during this period, the Mantis and Groot (Sapling) Tsums will come with 5 Luck (until 6/3 11:59PM PT)!
HP, ATK, and DEF will be boosted by 500 for Mantis and Groot (Sapling) for a limited time only (until 6/30 11:59PM PT)! (Boosts to Tsum Tsum will be removed after the deadline ends.)
Source: http://marveltsumtsumgame.net/en/news/20170530_2.html