Yes. With cascade, you can cast either face of a modal double-faced card, regardless of the back face's converted mana cost (C.R. 711.7, 711.7a, 702.84a). This is because cascade doesn't care about the card's characteristics at the moment you begin to cast that spell. See also Torrential Gearhulk, this thread, this thread (adventurer cards), and this thread (split cards). In the case of double-faced cards, the cascade ability cares whether the card's front face is "a nonland card whose converted mana cost is less than" that of the spell with cascade, but only at the moment cards are exiled with cascade (C.R. 702.84a, 711.4a).
Note, however, that cascade says "You may cast that card..."; thus, you can't play a land this way with cascade even if the card's back face is a land (C.R. 702.84a; see also C.R. 701.14a).
No. With cascade, you can cast either face of a modal double-faced card (C.R. 712.7, 712.7a, 702.85a). You can do so, however, only if the "resulting spell's mana value is less than" that of the spell with cascade (C.R. 702.85a).
When Bloodbraid Elf's cascade ability resolves, you exile cards from the top of your library until you exile a card with mana value less than 4 (since Bloodbraid Elf's mana value is 4). Say you exile Valki this way. Since the card's front face has mana value 2 (C.R. 712.4a), you stop exiling cards. Now you may choose to cast a spell from that card this way if its mana value is less than Bloodbraid Elf's of 4. You can choose to cast the Valki face this way (since its converted mana cost is less than 4), but not the Tibalt face, since the Tibalt face has mana value 7, so not less than 4.
Note, however, that cascade says "You may cast that card..."; thus, you can't play a land this way with cascade even if the card's back face is a land (C.R. 702.85a; see also C.R. 701.14a).
EDIT (Feb. 16): Edited in view of recent rule change.
EDIT (Mar. 7): Edited to conform to the text as it actually appears in the comprehensive rules.
EDIT (Sep. 19): Some rules were renumbered in the meantime.
EDIT (Nov. 12): Use updated terminology.
But Kaldheim release notes specifically call out that either side you cast must meet criteria, which tybalt obviously doesn't. Very confused.
The part of the article in question says: "If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect." (And the example is given of "cast[ing] artifact spells from your graveyard". Other examples include Haakon, Stromgald Scourge, Electrodominance, and Rishkar's Expertise.) This is not the kind of effect that cascade is, since it cares about a card's characteristics only while cards are exiled with that ability, not while it's being cast (review C.R. 702.84a).
EDIT (Feb. 16): Edited in view of recent rule change.
Ok thanks, hopefully they fix this rediculous loophole soon.
In fact, they just changed the rules for cascade, according to a recent announcement on the Banned and Restricted List. Now cascade means, in relevant part: "You may cast that card without paying its mana cost if the resulting spell's mana value is less than this spell's mana value" (C.R. 702.85a). Now, you can cast a face of a modal double-faced card with cascade only if its mana value is less than that of the spell with cascade.
EDIT (Feb. 17; Feb. 20): Edited.
EDIT (Mar. 7): Edited to conform to the text as it actually appears in the comprehensive rules.
EDIT (Sep. 19): One rule was renumbered in the meantime.
EDIT (Nov. 12): Use updated terminology.
Yes. With cascade, you can cast either face of a modal double-faced card, regardless of the back face's converted mana cost (C.R. 711.7, 711.7a, 702.84a). This is because cascade doesn't care about the card's characteristics at the moment you begin to cast that spell. See also Torrential Gearhulk, this thread, this thread (adventurer cards), and this thread (split cards). In the case of double-faced cards, the cascade ability cares whether the card's front face is "a nonland card whose converted mana cost is less than" that of the spell with cascade, but only at the moment cards are exiled with cascade (C.R. 702.84a, 711.4a).
Note, however, that cascade says "You may cast that card..."; thus, you can't play a land this way with cascade even if the card's back face is a land (C.R. 702.84a; see also C.R. 701.14a).
No. With cascade, you can cast either face of a modal double-faced card (C.R. 712.7, 712.7a, 702.85a). You can do so, however, only if the "resulting spell's mana value is less than" that of the spell with cascade (C.R. 702.85a).
When Bloodbraid Elf's cascade ability resolves, you exile cards from the top of your library until you exile a card with mana value less than 4 (since Bloodbraid Elf's mana value is 4). Say you exile Valki this way. Since the card's front face has mana value 2 (C.R. 712.4a), you stop exiling cards. Now you may choose to cast a spell from that card this way if its mana value is less than Bloodbraid Elf's of 4. You can choose to cast the Valki face this way (since its converted mana cost is less than 4), but not the Tibalt face, since the Tibalt face has mana value 7, so not less than 4.
Compare cascade with Torrential Gearhulk and see also this thread, this thread (adventurer cards), and this thread (split cards).
Note, however, that cascade says "You may cast that card..."; thus, you can't play a land this way with cascade even if the card's back face is a land (C.R. 702.85a; see also C.R. 701.14a).
EDIT (Feb. 16): Edited in view of recent rule change.
EDIT (Mar. 7): Edited to conform to the text as it actually appears in the comprehensive rules.
EDIT (Sep. 19): Some rules were renumbered in the meantime.
EDIT (Nov. 12): Use updated terminology.
This is not the kind of effect that cascade is, since it cares about a card's characteristics only while cards are exiled with that ability, not while it's being cast (review C.R. 702.84a).EDIT (Feb. 16): Edited in view of recent rule change.
EDIT (Feb. 17; Feb. 20): Edited.
EDIT (Mar. 7): Edited to conform to the text as it actually appears in the comprehensive rules.
EDIT (Sep. 19): One rule was renumbered in the meantime.
EDIT (Nov. 12): Use updated terminology.