Appendix C of this book contains the Secrets Deck—paper cards for you to photocopy and hand out to the players when their characters learn reliable information about Halaster and Undermountain.
Any character who spends at least four hours in the Yawning Portal is likely to overhear tall tales about the Mad Mage and his dungeon. Not all rumors are to be believed, however, and characters need keen ears and sharp insight to determine which stories have weight. Have each character make a DC 20 Intelligence (Investigation) check or Wisdom (Insight) check. If the check succeeds, the character learns a secret about Undermountain, and the player can draw a card from the Secrets Deck.
Players can hold onto these cards for later reference. When the characters meet an NPC in Undermountain who knows a fact, have one of the players draw another card from the Secrets Deck to determine what that NPC knows about Halaster and his dungeon.
Printable assets are available here.