Skeleton Key — ruling 1

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.

Skeleton Key — ruling 2

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.

Skeleton Key : règles, fonctionnement et interactions (MTG)

Skeleton Key is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Equipped creature has skulk. (It can't be blocked by creatures with greater power.) Whenever equipped creature deals combat damage to a player, you may draw a card. If you do, discard a card. Equip

How it works : Skeleton Key

Original English text — Wizards of the Coast / Scryfall

#1April 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 8, 2016· Wizards of the Coast
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.
No other card is named in this ruling.