Declare Krothuss and Rune-Scarred Demon as attackers.
Krothuss triggers, creating a token copy of Rune-Scarred Demon that is tapped and attacking.
The Rune-Scarred Demon token enters the battlefield, causing you to search your library for Beacon of Tomorrows and put it into your hand.
Cast Beacon of Tomorrows by paying {6}{U}{U}, causing you to take an extra turn after this one and shuffle Beacon of Tomorrows into your library.
Repeat each turn.

Infinite turns

Lock

Runo Stromkirk // Krothuss, Lord of the Deep

Rune-Scarred Demon

Beacon of Tomorrows

Declare Krothuss and Rune-Scarred Demon as attackers.
Krothuss triggers, creating a token copy of Rune-Scarred Demon that is tapped and attacking.
The Rune-Scarred Demon token enters the battlefield, causing you to search your library for Nexus of Fate and put it into your hand.
Cast Nexus of Fate by paying {5}{U}{U}, causing you to take an extra turn after this one and shuffle Nexus of Fate into your library.
Repeat each turn.

Combos with "Rune-Scarred Demon"