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.
In the fanfiction, the Howler might contain a riddle or a cryptic message. Voldemort is known for his devious mind. He could be leading Harry into a trap by giving him a seemingly innocent message that actually has a hidden meaning. For example, it could be about a place where Voldemort has set up an ambush for Harry and his friends. If Harry misinterprets the message, they could all be in great danger.