Primordial Gnawer

The mana value of a split card is determined by the combined mana cost of its two halves. If discover allows you to cast a split card, you may cast either half (as long as its mana value is less than or equal to the effect's discover value) but not both halves.

When you discover, you must exile cards. The only optional part of the ability is whether you cast the exiled card or put it into your hand.

If you can't cast the discovered card (perhaps because there are no legal targets for the spell), you'll put it into your hand.

If the discovered card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you discover an adventurer card, split card, or modal double-faced card, you might be able to cast that card with either set of characteristics depending on the effect's discover value. For example, if you discover 4 and reveal Galvanic Giant (an adventurer card from Wilds of Eldraine with a mana value of 4), you could cast Galvanic Giant, but not Storm Reading (its Adventure, which has a mana value of 7). If you discover 7 and reveal Galvanic Giant, you could cast either Galvanic Giant or Storm Reading.