As soon as he was spotted, however, Voldemort changed tactics. He left Barty to defend him and started attacking Severus. While the Potion Master was good, he too would not be able to stand up to his former Master indefinitely. When a bone-shattering curse slipped past his shield, he bit down a cry of agony. The next second, a wave of pure magic swept everybody off their feet. Casting a shield with the last of his strength – the pain was draining him rapidly – Severus watched the Dark Lord rise and approached his equal. In a surprisingly warm tone, he complimented the exhausted teenager, "Very good, now we merely need to channel this power correctly, and nobody will be able to stand against the three of you."