Who could have sent the howler to Voldemort in the 'voldemort gets a howler' fanfiction?

In the fanfiction, the howler could be from an unexpected source. Maybe it's from a long - forgotten enemy who has finally found a way to get to Voldemort. The howler's message could be a warning about a new threat that is coming his way, something that could even be more powerful than him. So Voldemort has to decide whether to ignore it or take it seriously.

It could lead to all sorts of chaos! Maybe Voldemort gets really angry and lashes out.

Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.

It could be one of his professors. If Teddy was constantly late to class or not doing his homework, a strict professor like Professor McGonagall might send a Howler to set him straight.

The Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.

Perhaps it's a taunt about Harry's power. Voldemort could be saying that all his efforts to defeat him are in vain because Voldemort is far more powerful. He might mock Harry's attempts at learning new spells and defenses, saying they won't be enough to stop him in the long run.

His mother could send the howler. In the wizarding world, mothers can be quite fierce when it comes to family reputation and she might be unhappy with Sirius's actions or friends.

It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.

Perhaps another student, like Hermione, could send the Howler in a very creative twist in the fanfiction. Maybe she has found out that Draco is planning something malicious against the Gryffindors and uses the Howler as a way to threaten him into stopping.

Hermione could be the sender. If Snape has made some unfair judgment in class against Gryffindor students, Hermione, with her strong sense of justice, might send a howler to him to voice her protest.