Yidris, Maelstrom Wielder
Oracle Text
Trample
Whenever Yidris, Maelstrom Wielder deals combat damage to a player, as you cast spells from your hand this turn, they gain cascade. (When you cast the 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.)
Card Rulings
11/8/2016 Spells you cast from your hand this turn will gain cascade even if Yidris leaves the battlefield.
11/8/2016 If Yidris’s triggered ability resolves more than once in one turn, spells you cast from your hand will gain cascade that many times. Each instance of cascade triggers separately; resolve one cascade trigger and the spell you cast from it before resolving the next cascade trigger.
11/8/2016 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.
11/8/2016 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.
11/8/2016 If a spell with cascade is countered, the cascade ability will still resolve normally.
11/8/2016 You exile the cards face up. All players will be able to see them.
11/8/2016 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. It can be countered. Since it’s cast from exile, Ydris’s ability won’t give the new spell cascade.
Just as creatures on the battlefield can gain/have abilities, so can spells being cast. In some cases, these abilities are not cumulative, such as trample/haste/lifelink for creatures or "split second"/"can't be countered" for spells. In other cases, abilities are cumulative, for both creatures and spells, such as multiple instances of Vampiric Sliver for creatures, or a cumulative reduction in spell cost by multiple Mycosynth Golems or multiple Cascade triggers gained by spells from multiple Maelstrom Nexus on the battlefield.
What makes this confusing is the wording on Yidris. The ability is broken into three distinct parts: the 1st ability trigger, the 2nd trigger point, and the abilities resolution. As you can see these parts are separated by commas. Commas in the rules text of triggered abilities always separate triggers, intervening "if" clauses, and ability resolutions. Yidris is unique in that it (to my knowledge) is the first card to have a secondary resolution check criteria that is not an intervening if clause, as shown as an example by Kryptnyt.
The 1st ability trigger of Yidris is simple enough. "Whenever X deals combat damage,". We've seen this plenty of times and know that whenever the creature deals combat damage multiple times, be it through double strike, multiple combat phases, or even on different turns, the ability will trigger once for each instance of combat damage.
The 2nd trigger point cares primarily about its wording, and here is where Kryptnyt's example falls a bit short. The etiquette for casting a spell is usually glanced over in many games of magic, as we've seen countless times players have tapped for the appropriate mana, and then moved the card from their hand to become a spell on the stack. In reality, a spell is announced as being cast, and then all subsequent costs necessary to pay for the spell are paid. The most recent reminder of this sequencing is "Emerge" cards. Creatures sacrificed to Emerge costs are on the battlefield up until the point when the spell is completely paid for and moved from a card in hand to a spell on the stack. Moving back to Kryptnyt's example, this is where the wording "Whenever you cast a spell" would fall short. By the time that ability has triggered, it is too late for the spell to gain Cascade. This is why Yidris uses the wording "As you cast spells." A clear analogy can be seen for the usage of this wording when examining creatures like Adaptive Automaton and Arsenal Thresher and spells with Kicker or Buyback.
Adaptive Automaton and Arsenal Thresher use the "As X enters the battlefield" to bypass periods when state-based effects would be checked. In the case of the Automaton, creatures of the chosen type immediately gain +1/+1 and it would be too late to kill an X/1 with a Darkblast. In the case of the Thresher, revealing any artifact cards from hand would cause it to enter the battlefield with +1/+1 counters already on it. It would never be seen as a 2/2 and could not be killed by a "Disfigure" when your opponent next gains priority.
In summary, Yidris' wording is due in large part to the hierarchy of trigger points for spells and permanents which looks loosely something like this.
1. As you cast a spell - (Buyback / Kicker, trigger when a spell is announced as cast. By this point you must have announced whether or not you intend to pay for these optional costs)
2. Effects from additional costs - (Madness from a spell discarded due to additional cost i.e. Tormenting Voice or "dies" triggers from a creature sacrificed to Emerge)
== At this point. The spell is on the stack ==
3. When(ever) you cast a spell (Cascade, Eldrazi "cast" effects, etc. These triggered abilities create priority)
== If the spell is a creature spell, isn't countered, and resolves ==
4. As X enters the Battlefield (Examples mentioned above, and other effects such as Clone and cards with Modular, so these cards are never seen as 0/0's which would summarily die to state-based effects.
5. When(ever) X enters the battlefield. (Too many examples to count! These triggered abilities create priority)
It wouldn't have worked if it had said "Whenever you cast a spell, if an opponent was dealt combat damage by Yidris, it gains cascade."
At least that's how I read it, but I'm no rules-ologist.