Cresting Mosasaurus

Destroy all tapped creatures. Until your next turn, whenever a creature becomes tapped, destroy it.

Whenever this creature enters or attacks, put a -1/-1 counter on up to one target creature. Creatures your opponents control with -1/-1 counters on them can't block.

First strike Dinosaur spells you cast have prowl 2R. (You may cast a spell for its prowl cost if you dealt combat damage to a player this turn with a creature with any of its creature types.)

Whenever a nontoken creature you control with power 4 or greater enters, populate. (Create a token that's a copy of a creature token you control.)

As an additional cost to cast this spell, sacrifice a creature with power 4 or greater. Search your library for a Dinosaur creature card, put it onto the battlefield, then shuffle. It gains indestructible until your next turn.

(As this Saga enters and after your draw step, add a lore counter.) I — For each opponent, up to one target noncreature artifact they control becomes a 0/4 Wall artifact creature with defender for as long as you control this Saga. II — Create a 3/3 green Dinosaur creature token with trample. It gains haste until end of turn. III — Destroy all Walls. Exile this Saga, then return it to the battlefield transformed under your control.

Partner with Owen Grady, Raptor Trainer (When this creature enters, target player may put Owen into their hand from their library, then shuffle.) For each kind of counter on Blue, Loyal Raptor, each other Dinosaur you control enters with a counter of that kind on it.

At the beginning of your end step, if a creature died this turn, create a tapped token that's a copy of this creature.