If this card was Special Summoned by the effect of a "Gladiator Beast" monster, then at the end of any Battle Phase in which this card attacked or was attacked: Special Summon 2 "Gladiator Beast" monsters from your Deck, except "Gladiator Beast Secutor".
Unlike other gladiator beasts, Secutor doesn't shuffle into the deck, so if you want to activate his effect, he needs to be the only monster you have on the field to leave room for the two you're trying to special summon.
It only triggers if he's summoned by a "Gladiator Beast" effect, not if normal summoned or summoned via Test Ape.
The conditions to use his effect properly are pretty limited, but if you have an Impenetrable Attack or even Molting Escape ready for him then you can very quickly flood the field with GBs and fuse one of their more powerful variants to win.
Secutor is definitely broken. Summoned him from deck off g beast effect. used impenetrable defense to eat an attack on him. hit activate effect. then the turn just ends. look in the logs at the top right and it says his effect activated but nothing happened. Sending a bug report.
To Activate his Effect. You must make sure you have no other monster other than Secutor on the field to activate the special summoning of two monsters. Best cards to combo would be impenetrate and half shut