Phase-Dependent Mandatory Triggers Do Not Repeat Forever

Some older effects activate at a phase boundary or during a phase because a condition is true. Do not assume that mandatory means the effect will keep creating new Chains over and over in the same phase until something changes. Mandatory means the effect must activate at its proper opportunity; it does not erase once per turn, once-per-copy, or once-per-phase handling for that phase-dependent trigger.

Mandatory is not infinite

A mandatory Trigger Effect still follows its own timing and repeat limits. If an old card has a phase-dependent trigger that activates once during the Standby Phase, End Phase, or another specified window, a negation of that one activation does not automatically make the same copy activate again in a fresh Chain during that same phase.

This is especially important against once-per-chain automatic negators such as Light and Darkness Dragon. A mandatory effect negated by Light and Darkness Dragon does not necessarily keep re-triggering in separate Chains until Light and Darkness Dragon loses enough ATK/DEF to stop negating. Ask whether that trigger has already used its phase opportunity.

Worked example

Reptilianne Naga has an old phase-dependent effect that can reduce an opponent's monster's ATK.

"Will Reptilianne Naga keep activating in separate Chains against Light and Darkness Dragon until Light and Darkness Dragon runs out of ATK/DEF and cannot negate?"

— No. Treat the phase-dependent trigger as one activation opportunity for that copy in that phase. Light and Darkness Dragon can negate that activation; the same copy does not keep re-activating indefinitely in separate Chains just because the effect was mandatory.

Judge calls to watch for

• Mandatory means "must activate at the proper opportunity," not "repeat forever."
• Do not convert a phase-dependent trigger into an infinite sequence of separate Chains.
• If the card has modern once-per-turn text, use that text. If it is an older phase-dependent trigger without clean modern text, answer cautiously and preserve the phase opportunity / per-copy framing.
• Against Light and Darkness Dragon, ask whether the same effect has already had its activation opportunity before assuming repeated negations.

Sources

• Reptilianne Naga / Light and Darkness Dragon community and legacy ruling handling; no official unified TCG database source was found in the local corpus for this exact old-card pattern, so keep the answer narrow and avoid broad claims about every mandatory trigger.
• General trigger timing: Konami Fast Effect Timing and rulebook phase structure.