Farbog Revenant — 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.

Farbog Revenant — 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.

Farbog Revenant : règles, fonctionnement et interactions (MTG)

Farbog Revenant 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

Skulk (This creature can't be blocked by creatures with greater power.) Lifelink (Damage dealt by this creature also causes you to gain that much life.)

How it works : Farbog Revenant

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.