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Skulk matters only as blockers are chosen. Modifying either creature's power after blockers are chosen won't cause the attacking creature to become unblocked.

The upkeep step is before the draw step, after the untap step. If you have no cards in hand, you can't draw for the turn and then discard that card to draw a new one because Forgotten Creation's last ability will already have triggered and resolved.

If you cause a creature to have 0 power or less, use the actual value (which may be negative) to determine whether it can block or be blocked. A creature with skulk and 0 or less power most likely won't be blocked, but it won't deal combat damage and won't trigger any abilities that trigger when combat damage is dealt.