This card's effect that places a Spell Counter on it each time a Spell Card is activated does not start a Chain. (This effect can be applied during the Damage Step.)[1]
This card's effect that Special Summons a Level 7 or higher Spellcaster monster from your Deck starts a Chain. (It does not target. Removing 6 Spell Counters from your field is the cost to activate this effect.)[1]
You can activate this effect optionally after damage calculation when a Spellcaster monster on your field is destroyed by battle.[1]
A: The effect of "Endymion's Lab" that Special Summons a Level 7 or higher Spellcaster monster from the Deck activates and resolves after damage calculation. Furthermore, the effect of "Endymion, the Mighty Master of Magic" that adds a Normal Spell from your Deck to your hand activates at the end of the Damage Step, after it has left the field. In this scenario, since you removed all Spell Counters from "Endymion, the Mighty Master of Magic" when activating the effect of "Endymion's Lab", you cannot activate the effect of "Endymion, the Mighty Master of Magic" at the end of the Damage Step.[4][3]