MTG Salvation

So, if I give him double strike or hit an opponent again via Relentless Assault, does that give my spells multiple instances of cascade?

This was a real head scratcher, but after sifting through quite a bit of rules, the TL DR of the question of Yidris creating multiple instances of Cascade on a card is, (drum roll please) YES, additional instances of Yidris dealing combat damage will create multiple instances of Cascade on spells you cast that turn.

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)

If you look at Maelstrom Wanderer, it has "Cascade, cascade" which proves that cascade stacks. This seems to work just fine. Multiple instances of this guy's triggered ability should operate seperately.
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."

I'd assume from the way its worded, in this case multiple instances of cascade wouldn't stack. The way it says "they gain cascade" to me suggests that it only gives a spell Cascade if it doesn't already have it, not that it gives +1 cascade, for lack of a better way to put it. So if this effect was active, and you cast Maelstrom Wanderer, you'd only get 2 cascade triggers, not 3.

At least that's how I read it, but I'm no rules-ologist.

It does seems like they gain cascade again, so give him double strike and play stuff that gives you extra combat phases and attack forever!

Yap, double strike seems to be redundant here. It has trample, which is certainly better for forcing damage.

I would assume since it says "as you cast spells from your hand this turn" that it wouldn't matter how many times you hit them.

Eye of the storm anyone?

Dang it, didn't notice it was hand only, well there goes a lot of broken combos.

What happens if I cascade a lotus petal Cmc doesn't go below zero so what happens.

A Judge told me that the only thing technically lower than 0 is suspend cards that only have the option to suspend. So I'm not 100%, but if you say, cast ornathopter and cascaded into a suspend card, the suspend card would be cast on the spot since it basically has no initial cost (kind of like a land if lands were spells).

The assumption is for cascading for Cmc 0