30291 Anacondrai Battle Mech is a LEGO Ninjago set released in 2015. It includes Sleven and and a mini mech. It is possibly chosen to oppose Jay's Nano Mech in combat.
Alongside 30080 Ninja Glider, the Anacondrai Battle Mech is the only polybag to make an appearance in the show. Additionally, it's the only one to be mentioned on the official website, namely in Sleven's official description.