The Hokages have been reanimated by orochimaru. but i didn't see the jutsu being released. did i miss something? where are the reanimated hokages now?
I'm not quite sure which instance you are talking about. If it is the one during the 4th shinobi war then Balance Breaker's answer would be right, but if you are talking about part one when Orochimaru infiltrates the chunin exam finals, then I can help.
Hiruzen Sarutobi (3rd Hokage) used shadow clones and the reaper death seal to seal the 1st and 2nd Hokage away right before using his own body and the reaper death seal to seal Orochimaru's arms. Later, in the 4th shinobi war, someone (not sure who) took the Shinigami mask from the Uzumaki clans storage temple which when worn allows the wearer to perform the "reaper death seal: release" and took the souls of the Hokages back out of the reaper.
Minato Namekaze (4th Hokage) couldn't be reanimated by Orochimaru the first time because he already used the reaper death seal to seal himself and half of the nine tails chakra away.