This effect is Mandatory. It can (and must) activate in any Step and any Phase, including Damage Calculation, and even in response to a effect triggered by an End Phase discard.
If 2 "Doomcaliber Knight" are on the field, when a monster effect is activated, both are tributedm, and both "try" to negate that monster effect. however, only the one with a Chain Link directly adjacent to that monster effect in the Chain (the first to activate) will actually negate that Monster Effect.
The effect of "Doomcaliber Knight" can activate during the Damage Step.
The effect of "Doomcaliber Knight" is a Quick Effect. When an Effect Monster's effect is activated, the effect of "Doomcaliber Knight" activates in a Chain to that effect. This effect does not target.
Tributing "Doomcaliber Knight" is a cost.
The effect of "Doomcaliber Knight" will not activate while "Mask of Restrict" is active, because it cannot be Tributed.
If the effect of "Kuriboh" is activated to negate the Battle Damage from "Doomcaliber Knight", "Doomcaliber Knight" is Tributed to negate the effect of "Kuriboh". Damage has already been calculated, so if "Doomcaliber Knight" was attacking a monster with equal or lower ATK that monster will be destroyed, and Battle Damage will be inflicted as normal.
If the effect of "Doomcaliber Knight" activates while "Skill Drain" is face-up on the field, its effect is not negated because it is not on the field when its effect resolves.
If "Doomcaliber Knight" is destroyed by battle and an Effect Monster's effect activates after damage calculation ("D.D. Warrior Lady", etc.), the effect of "Doomcaliber Knight" still activates.
If an Effect Monster's effect is activated while multiple copies of "Doomcaliber Knight" are face-up on the field, they all activate and are Tributed. Their effects are chained following the rules for simultaneous effects . Only the last "Doomcaliber Knight" to resolve will be applied, and the effects of the other copies of "Doomcaliber Knight" disappear.
Player B activates an Effect Monster's effect. Both of Player A's "Doomcaliber Knight" activate and form a Chain:
Effect Monster –> "Doomcaliber Knight" A –> "Doomcaliber Knight" B
Only "Doomcaliber Knight" A negates the Effect Monster's effect.
When multiple Effect Monster effects activate simultaneously, the effect of "Doomcaliber Knight" is chained after determining the order of the Effect Monster effects and will negate the Effect Monster's effect that was placed last on the Chain.
The effect of "Sangan" is Chain Link 1 and the effect of "Witch of the Black Forest" is Chain Link 2. The effect of Player B's "Doomcaliber Knight" is chained, and it will negate the effect of "Witch of the Black Forest".
Player A activates "Card Destruction". When both copies of "Goldd, Wu-Lord of Dark World" are discarded, their effects start a Chain. Player A is the turn player, so his "Goldd, Wu-Lord of Dark World" is Chain Link 1 and Player B's is Chain Link 2. The effect of "Doomcaliber Knight" is chained, and it will negate Player B's "Goldd, Wu-Lord of Dark World".
The effect of "Doomcaliber Knight" is chained, negating the effect of "Raiza the Storm Monarch" effect and destroying it.
The effect of "Sasuke Samurai #4" is Chain Link 1 and "Neo-Spacian Grand Mole" is Chain Link 2. The effect of "Doomcaliber Knight" is chained, and it will negate and destroy "Neo-Spacian Grand Mole".
If an Effect Monster's effect that is not optional activates, even if it would not do anything when it resolves, it will still be negated by "Doomcaliber Knight".
The Flip Effect of "Magician of Faith" activates and the effect of "Doomcaliber Knight" activates in a Chain to it.
If your "Doomcaliber Knight" attacks an Effect Monster with a non-continuous effect and destroys it while you control "Dark Ruler Ha Des", when the Effect Monster's effect activates the effect of "Doomcaliber Knight" is still chained.
If the effect of "Doomcaliber Knight" would destroy a monster on the field, "My Body as a Shield" can be chained.
Even if "Book of Moon", etc., is chained to the effect of "Doomcaliber Knight" and flips the Effect Monster face-down, the Effect Monster's effect is still negated by "Doomcaliber Knight" and it is still destroyed.
If "Light and Darkness Dragon" and "Doomcaliber Knight" are face-up on the field when a Spell or Trap Card is activated, the effect of "Light and Darkness Dragon" is chained to the Spell or Trap Card and the effect of "Doomcaliber Knight" is chained to "Light and Darkness Dragon".
If "Doomcaliber Knight" attacks an Attack Position "Reflect Bounder", when the effect of "Reflect Bounder" activates the effect of "Doomcaliber Knight" is chained. "Doomcaliber Knight" negates the effect of "Reflect Bounder" and destroys it. Damage Calculation is not conducted.
If the effect of "Crystal Beast Ruby Carbuncle" is activated when it is Special Summoned and the effect of "Doomcaliber Knight" is activated, "Doomcaliber Knight" will negate the effect of "Crystal Beast Ruby Carbuncle" that Special Summons 'Crystal Beast' monsters and destroy it. The player controlling "Crystal Beast Ruby Carbuncle" can place it into their Spell and Trap Card Zone.