the emblem was teased about 3 times so far before this episode on which we saw it in full
it is connected to whatever is possessing people and running off their hatred
It is strange that it is shown with Boruto.
One hypothesis is that Denki' s cursed chakra infected Boruto, and when he tried to summon a snake, the chakra woke up and the beast that cursed Denki has been summoned.
Another hypothesis is that Boruto stole the scroll where that beast has been sealed, once he tried to summon it, the symbol appeared.But that implies that the beast has been sealed in the scroll days ago, considering that the beast was free when it cursed Denki.