The Soul-sucking Technique.
It was a secret technique passed on to Ou Yangming by the little phoenix, and it could be used to perfectly control a spirit beast.
Once the secret technique was used, the young fellow would not need to worry about the Eight-armed Giant Beast revolting. This was because the technique could not only control the spirit beast's life with ease, but it could also give spiritual suggestions.
The longer the technique was used, the more obedient the spirit beast. As such, the beast would unlikely rebel.
Needless to say, not all spirit beasts were qualified enough to be on the receiving end of the secret technique. The technique would expand a spirit beast's potential, giving it a chance to reach a higher boundary.
If it was not because they were in the upper realm where there were no stronger spirit beasts to choose from, the little bird might not have been willing to release the secret technique.