He didn't promise them anything. However, with his little words here, he managed to shift the attention of many disciples towards the ongoing war between the academy and good clans on one side, and the dark spirit master community.
Doing this would give a little boost to the ongoing battle, helping the academy and clans to bring more harm to the enemy.
William also wanted to slowly grow his authority in the academy. If a few disciples followed his words and started to help, then their achievements would always be linked to him.
Like this, he'd not only help the academy in the current war, but also create a name for himself.
As for adding up the other conditions, or hints, he did it to cover up his true goals. So, anyone thinking about his words later on wouldn't be able to guess his true aim correctly.
Only those knowing him quite well, like the academy headmaster or the Long clan's patriarch and elders, would realise his true goal from saying these words.
