The ③ effect:
It can't be activated unless there is a "Gladiator Beast" monster in your Deck.
It can activate and resolve while "Skill Drain" is applying (without being negated).
It can be negated by "Royal Oppression" (If it is, this card remains in the Deck (it is not destroyed)).
"Battling" requires this card to be involved in Damage Calculation (If this card's attack is negated, the ③ effect can't activate).
If multiple "Gladiator Beast" effects are activated at the end of the Battle Phase, they don’t go on the same Chain (they each activate, one after another, each on their own Chain). Vs "Rivalry of Warlords"/"Gozen Match":
If you have a Gladiator Beast of the same Type/Attribute in your Deck, you can activate the ② effect. If you do, you can Summon any Gladiator Beast of any Attribute. Darius' Negation:
The target's effects aren't negated if Darius is Set or leaves the field (except by "Bottomless Trap Hole").
The target's effects aren't negated if the target is Set or leaves the field.
If Darius is flipped face-down, the target won't be shuffled into the Deck if Darius leaves the field.
The 'Gladiator Beast' monster Special Summoned by "Gladiator Beast Darius" is continually targeted by its effect.
Returning the Special Summoned 'Gladiator Beast' monster to the Deck does not start a Chain.
If the Special Summoned 'Gladiator Beast' attacks or is attacked, it can be returned to the Deck to activate its effect, but the effect will be negated by "Gladiator Beast Darius".
While "Skill Drain" is active, if "Gladiator Beast Darius" is removed from the field, the Special Summoned 'Gladiator Beast' monster will remain on the field. If the Special Summoned 'Gladiator Beast' monster attacks or is attacked, and it is returned to the Deck to activate its effect, the effect will not be negated because the effects of "Gladiator Beast Darius" are being negated by "Skill Drain".
When "Gladiator Beast Darius" is flipped face-down, its effect disappears and the effects of the Special Summoned 'Gladiator Beast' monster are no longer negated. If "Gladiator Beast Darius" is removed from the field while face-down, the Special Summoned 'Gladiator Beast' monster will not be returned to the Deck.
If "Bottomless Trap Hole" is chained when "Gladiator Beast Darius" is Special Summoned and activates its effect, the selected 'Gladiator Beast' monster is still Special Summoned and its effects are not negated.