MTG Salvation

• Haakon, Stromgald Scourge could allow you to begin casting some adventures which are on creatures that have the Knight type, such as Murderous Rider. This is because Haakon lets you cast the Knight card, allowing you to place it on the stack (601.2a).
• When the spell is placed on the stack, you then have to make a choice, of whether you are casting it as a Creature, or as an Adventure (likely to be included in 601.2b). At this point, you may choose to cast the card as an adventure.
• However, once the process arrives at 601.2e, where the game checks for legality, we have a problem. Since you have chosen to cast the card as an Adventure, it only has the characteristics of the Adventure - notably, it is no longer a creature, and most problematically - not a Knight anymore. Haakon only lets you cast Knights from the graveyard, and now that this card is no longer a Knight card - we have a problem. This is an illegal move, and the game will rewind to the point right before you announced an attempt to cast this spell.

601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell includes proposal of the spell (rules 601.2a–d) and determination and payment of costs (rules 601.2f–h). To cast a spell, a player follows the steps listed below, in order. A player must be legally allowed to cast the spell to begin this process (see rule 601.3). If a player is unable to comply with the requirements of a step listed below while performing that step, the casting of the spell is illegal ; the game returns to the moment before the casting of that spell was proposed (see rule 721, “Handling Illegal Actions”).

601.2a To propose the casting of a spell, a player first moves that card (or that copy of a card) from where it is to the stack. It becomes the topmost object on the stack. It has all the characteristics of the card (or the copy of a card) associated with it, and that player becomes its controller. The spell remains on the stack until it’s countered, it resolves, or an effect moves it elsewhere.

601.2b If the spell is modal, the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the spell (see rule 702.46), they reveal those cards in their hand. If the spell has alternative or additional costs that will be paid as it’s being cast such as buyback or kicker costs (see rules 118.8 and 118.9), the player announces their intentions to pay any or all of those costs (see rule 601.2f). A player can’t apply two alternative methods of casting or two alternative costs to a single spell. If the spell has a variable cost that will be paid as it’s being cast (such as an {X} in its mana cost; see rule 107.3), the player announces the value of that variable. If the value of that variable is defined in the text of the spell by a choice that player would make later in the announcement or resolution of the spell, that player makes that choice at this time instead of that later time. If a cost that will be paid as the spell is being cast includes hybrid mana symbols, the player announces the nonhybrid equivalent cost they intend to pay. If a cost that will be paid as the spell is being cast includes Phyrexian mana symbols, the player announces whether they intend to pay 2 life or the corresponding colored mana cost for each of those symbols. Previously made choices (such as choosing to cast a spell with flashback from a graveyard or choosing to cast a creature with morph face down) may restrict the player’s options when making these choices.

{... (rules for selecting targets) ...}

601.2e The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 721, “Handling Illegal Actions”).

601.3E
We had some contradictory rulings going on about split cards versus morph and when you look at what sets of characteristics. Adventures gave us the reason to shore up these rules. This isn't a change for morph, but it does change some answers for split cards: if you're allowed to cast a spell with a certain mana cost or color, look only at the half you're casting. Kari Zev's Expertise will allow you to cast Beck (but not Call).

But stop right there before you get too excited—you can't cascade into Beck // Call with Shardless Agent, since cascade finds a card with a certain converted mana cost and then says that you can cast the card you found. Beck // Call's converted mana cost is 8, and 8 > 3, so you're still out of luck with cascade.

601.4
The old 601.3 rule started off by talking about whether it was legal to begin casting, and then all of its subrules went into details. But then the main rule continued on talking about what if it became illegal! I broke that out into its own rule and added clarity that becoming illegal after the proposal is done doesn't matter.

601.3e If a rule or effect states that only an alternative set of characteristics or a subset of characteristics are considered to determine if a card or copy of a card is legal to cast, those alternative characteristics replace the object’s characteristics prior to determining whether the player may begin to cast it.
Example: Garruk’s Horde says, in part, “You may cast the top card of your library if it’s a creature card.” If you control Garruk’s Horde and the top card of your library is a noncreature card with morph, you may cast it using its morph ability.
Example: Melek, Izzet Paragon says, in part, “You may cast the top card of your library if it’s an instant or sorcery card.” If you control Melek, Izzet Paragon and the top card of your library is Giant Killer, an adventurer creature card whose Adventure is an instant named Chop Down, you may cast Chop Down but not Giant Killer. If instead you control Garruk’s Horde and the top card of your library is Giant Killer, you may cast Giant Killer but not Chop Down.