Missing (according to one theory, it might be a imaginary sword.)
In the famous war record Gikeiki, it said that Iwatooshi was smithed by Sanjou Munechika. However some argued that it is highly unlikely for Sanjou Munechika to smith a halberd as the blacksmith is known for his unique crescent shaped katana and a halberd doesn't seem to fit his style at all.
Iwatooshi was wielded by Musashibou Benkei, who took the task of guarding the Gojo bridge at Kyoto, aiming to collect as many katanas as he could from samurais who wanted to pass through the bridge.
Musashibou Benkei eventually collected up to 999 katana. However one day on his 1,000th battle, he lost to Minamoto no Yoshitsune. Since then he pledged loyalty to Yoshitsune and became his follower.
In the final years of Benkei and Yoshitsune, when Yoshitsune was betrayed by Fujiwara no Yasuhira and decided to commit seppuku, Benkei stood at the bridge outside of Yoshitsune's mansion. Yoshitsune ordered Benkei, his last surviving follower to guard the mansion as he commit seppuku.
As Musashibou Benkei used to be a yamabushi, therefore you can see Iwatooshi in Touken Ranbu clad in monk-like clothing.