The Phoenix Stone is a magical stone that was given to Bek Ohmsford by the King of the Silver River. The King of the Silver River told Bek to throw it to the ground when he was in danger: when used the stone would shatter on impact, meaning that it could only be used once.
On the journey to Parkasia, Bek gave the Phoenix Stone to Ahren Elessedil because Ahren was struggling with his fear and had no magic of his own to rely on at the time. Walker Boh warned Bek that he should not have given away the stone and urged Bek to try and get it back from Ahren as soon as possible. However, Bek and Ahren were separated before Bek could do anything about it.
However, Ahren made good use of the Phoenix Stone to save himself and the seer Ryer Ord Star from the artificial intelligence Antrax. When Ahren used the Phoenix Stone, a cloud of mist surrounded himself and Ryer, making them invisible and intangible to their enemies. Instead of escaping, Ahren and Ryer chose to use the stone's cloaking powers to look for Walker Boh and save him from Antrax, which was holding him captive. They were successful in their efforts to free Walker and once free, Walker was able to destroy Antrax.