The Pilgrim

The Pilgrim is the deadliest assassin of the Time Masters and is sent to kill Rip Hunter and his team. She is an antagonist in Legends of Tomorrow.

History
After the Hunters have failed to kill Rip Hunter and his team, the Time Masters issue Omega Protocols on every team member. While dying, one of the Hunters tells Mick Rory about the Omega Protocols and that the Pilgrim will now be coming for them. Back on the Waverider, Rory warns the rest of the team about the Pilgrim. Rory and Rip explain to the rest that the pilgrim is a Time Master assassin and that she, as every team member is a skilled fighter, will be targeting younger and more vulnerable versions of them.

Indeed she is seen in 1990, killing a younger version of presumably Leonard Snart.