The Heir costs 1B to cast for a 2/1, which is pretty unexciting on its own. But with the return of the Madness mechanic, discarding a card isn’t quite the cost that it usually is. Cards with Madness can be cast for less than normal as they are discarded. Actually, in many ways, the Heir to the Night can make an appearance more consistently than Delver’s flip side. With Delver, you have to have an instant or sorcery card on top of your deck during your upkeep. With the Heir, you can flip it whenever you want by discarding a card.
You may wonder why it says that you can only activate its flip ability once per turn. This is so you can’t just discard every card in your hand and trigger Madness multiple times. If you have multiple copies of Heir of Falkenrath on board, you can still trigger each one once. You only need to have a couple of good Madness cards in hand to do some serious work. In some respects, this is better than Delver of Secrets in the right deck. You just have to build around this uncommon Vampire a bit differently than you would around the Delver of Secrets.
Delver of Secrets isn’t going anywhere in Modern and Legacy, and especially Pauper being that it’s only a common. But Heir of Falkenrath and her flipside Heir of the Night could turn out to be a powerful creature. She’s super good in a Madness-happy Limited environment, but she’s definitely strong enough for Constructed, too.