A Howler is a magical letter that is delivered in a scarlet-red envelope. Upon arrival, it may be observed to be smoking faintly, hinting at the potent magic contained within. The primary purpose of a Howler is to convey a message in the sender's own voice, which has been magically amplified to a deafening volume. If a recipient hesitates or refuses to open a Howler, it will eventually explode and shout its message even more loudly than it would have otherwise. Once the message has been delivered in its entirety, the letter and envelope burst into flames and are reduced to ash, leaving no physical trace behind.
Howlers are enchanted to deliver a verbal message with extreme volume and force. They serve as a method of expressing immense anger, frustration, or displeasure, often with the intent of publicly shaming the recipient. The sender's voice is perfectly replicated, though magnified to an incredible degree, ensuring that everyone in the vicinity can hear the message. The enchantment includes a self-destruct mechanism, causing the Howler to burn up after its purpose is fulfilled. This prevents the message from being replayed or kept. The explosive consequence of ignoring a Howler serves as a powerful deterrent, forcing the recipient to face the sender's wrath immediately. While typically used for scolding, they can also be modified for malicious purposes, as seen when one sent to Hermione Granger contained harmful Bubotuber pus.
Howlers appear at several key moments throughout the series, often for dramatic or comedic effect.