Lathliss, Dragon Queen + Where Ancients Tread interaction

Question

Recently I constructed a Lathliss, Dragon Queen EDH deck, and just last night was at my LGS for some casual play.

At some point in the game I had on the battlefield a Lathliss, Dragon Queen and Where Ancients Tread, and was about to cast Scourge of Valkas.

My thought process was, if I activate Lathliss' ability to give my Dragon creatures +1/+0 then when Scourge of Valkas enters the battlefield it would enter as a 5/4, and therefore trigger Where Ancients Tread's ability.

The table seemed to conclude Lathliss' ability only applied the buff to Dragon creatures I currently controlled on the battlefield.

I'm confused, because it appears Lathliss' ability would apply to all Dragons, much the same as having Honor of the Pure on the field while casting a white creature with 2/2 would not cause Mentor of the Meek to trigger because the creature entering would actually be a 3/3, due to the added buff from Honor.

Could I have some clarification on this? Was my table right in that the interaction was not valid?

Answer 1

The table is correct. Lathliss will not buff Dragons that enter the battlefield after the ability has resolved.

Lathliss' ability produces a continuous effect that buffs your Dragons until the end of the turn. Part of generating this effect involves determining the set of objects that it will affect. From the Comprehensive Rules (emphasis mine):

611.2c If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won’t change.

Since Lathliss' ability has already resolved before you play Scourge of Valkas, Scourge won't be affected by that ability.

---

The situation with Honor of the Pure plays out differently, because its effect is produced by a static ability. Again from the Comprehensive Rules:

611.3a A continuous effect generated by a static ability isn’t “locked in”; it applies at any given moment to whatever its text indicates.

611.3b The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.

So Honor of the Pure will buff any white creature that enters the battlefield, as well as any creature that becomes white, or any white permanent that becomes a creature. Conversely, if a creature somehow stops being white, it immediately loses the Honor bonus.

Answer 2

You cannot use Lathliss's ability to make Scourge of Valkas trigger Where Ancients Tread. The other players were correct that Lathliss's ability only applies to dragons that you control when the ability resolves.

The key difference between Lathliss's ability and the effect of Honor of the Pure is that Lathliss's ability is a one-shot ability that creates a continuous effect, and Honor of the Pure is a static ability that creates a continuous effect. That means that they are governed by these two different parts of the continuous effects rules:

611.2c If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won’t change. (Note that this works differently than a continuous effect from a static ability.) [...]

611.3a A continuous effect generated by a static ability isn’t “locked in”; it applies at any given moment to whatever its text indicates.