Someone from the past will return in the upcoming episode of CW's "Arrow" season 6, and she bears some bad news for the people of Star City. In episode 16, titled "The Thanatos Guild," a mysterious heirloom will be the center of conflict as the League of Assassins are on a mission to steal that item, which could end up as bloody.

According to the latest promo for "Arrow" season 6 episode 16, Nyssa Al Ghul (guest star Katrina Law) will return to Star City to personally warn the people about the arrival of the League of Assassins. Specifically, Nyssa wants Thea (Willa Holland) to be extra careful, given that the League of Assassins is targetting a mysterious object that was last seen in the hands of Thea's father, Malcolm (John Barrowman). Even though Thea may not have what the League of Assassins are after, the group believes that it is in her possession, or she has an information regarding its whereabouts.

It looks like the League of Assassins is determined to get the item, putting Thea's life at risk. Based on the teaser, the item is a large golden box, and the group led by Athena (guest star Kyra Zagorsky) is willing to kill anyone for it. No one has an idea of what the mysterious box is for, probably except the League of Assassins who is aware of its importance to go all the way to Star City. Although, it could be safe to assume that the item brings harm, knowing that it belonged to Malcolm.

Nevertheless, Thea appears to be unbothered in spite of this new threat in her life, and Nyssa will convince her to take the situation seriously. Aside from Thea, Nyssa will also have a moment with Oliver Queen (Stephen Amell), as the latter is conflicted by the recent happenings in Star City.

"Arrow" season 6 has entered a two-week break after the last episode. The show will return on Mar. 29, Thursday, at 9 p.m. ET on The CW.