Apex Devastator
Creature — Chimera Hydra
Cascade, cascade, cascade, cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom of your library in a random order. Multiple instances of cascade each trigger separately.)
Illustrated by
Commander Legends#217MythicEnglishLegality
Rulings
Each instance of cascade triggers and resolves separately. The spell you cast due to the first cascade ability will go on the stack on top of the second, third, and fourth cascade abilities. That spell will resolve before you exile cards for the second cascade ability. The spell you cast for the second will resolve before you exile cards for the third, and so on.
Each of Apex Devastator’s four cascade abilities will look for a nonland card with converted mana cost less than 10 (Apex Devastator’s converted mana cost). This doesn’t change even if one or more of the spells you cast because of those cascade abilities has cascade itself. Each of those additional cascade abilities, if any, will refer to the converted mana cost of the spell that caused it to trigger. In other words, each cascade ability cares only about the spell that caused it to trigger, the stack can get messy, and we wish you luck.
The cascade ability triggers when you cast the spell, meaning that it resolves before that spell. If you cast the card exiled by the cascade ability, that spell will go on the stack above the spell with cascade.
When the cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not you cast the last card exiled.
Countering the spell with cascade has no effect on the cascade ability.
You exile the cards face up. All players will be able to see them.
You don’t have to cast the last card exiled by a cascade ability. If you choose to do so, you’re casting it as a spell. This means that it can be countered, and appropriate abilities trigger (such as if that spell also has cascade).
If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the spell.
If the card has 
in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
The converted mana cost of a spell with cascade is determined only by its mana cost, no matter what the total cost to cast the spell was.