This ability can be found on equipment cards, and it effectively means that those equipment cards come in, create a 2/2 creature token and automatically attach themselves to that token
For all the times you do not want your vampires dead, a little Accursed Duneyard is great to remind your opponents about regeneration and how hard it is to kill vampires
For example, in Simic infect, a one or two drop infect creature like Glistener Elf or Blighted Agent can swing at naked defenses or unblockable with the Agent, then turn into a powerful much faster kill with this equipped
But, of course, that assumes you can find the boxes in the first place
The Secret Missions and Themed Collections is one such event, where you need to collect certain cards and complete some hidden missions
Free to play, by the way