Eva Levante is an NPC that shows up in Destiny 2 from time to time, normally for special events, but she can always be found in the same place. When an event involving Eva is live in the game, you can simply load in at the main spawn point in the Tower, right beside the Cryptarch, and she will be there waiting for you.
At the moment, the active event involving Eva is the Festival of the Lost. This revolves around the Haunted Forest and going on runs to try and collect Candy and Chocolate Strange Coins so that you can get nice rewards from Eva.
The runs are limited to 15 minutes, and you will need to try and clear as many segments as you can, taking out a boss at the end of each one. Enemies will drop Candy, and you will also be able to complete bounties for Eva to earn Chocolate Strange Coins.
To start the event, just speak with Eva and you will get to pick a Mask. You will need to wear this if you want to go on runs through the Haunted Forest or earn a new resource called Cipher Decoders that you will need to open chests there. Eva will send you to talk to the Spider on Tangled Shore, and he will send you in search of a Cipher Decoder. This means you will need to finish a Strike, Crucible match, or Gambit match while wearing the Mask that Eva gave you.
After that is complete, you can then load into the Haunted Forest from the Director to go on your runs.