2018-10-05 : Because tokens aren’t cards, they never count for undergrowth abilities.

2018-10-05 : Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.

2018-10-05 : If you have no creature cards in your graveyard, the target creature gets -0/-0, and it will still be exiled if it would die this turn.

2018-10-05 : Necrotic Wound’s replacement effect will exile the target creature if it would die this turn for any reason, not just immediately after Necrotic Wound resolves.

2018-10-05 : The value of X is determined only as the undergrowth ability resolves. If the number of creature cards in your graveyard changes later in the turn, the target creature is unaffected.