Petra gave you a talisman that will allow you to enter the Dreaming City. Inspect it in your inventory to begin a quest to mend and charge it.
"This bauble Petra gave you… it's missing pieces. I know because I've seen 'em before around the Shore. Seen folk kill for 'em, too. If I had one, I'd stash it in a Lost Sector to keep it safe. You understand me?" —The Spider