Arcbound Mouser 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

Lifelink Modular 1 (This creature enters with a +1/+1 counter on it. When it dies, you may put its +1/+1 counters on target artifact creature.)

Complete guide

Arcbound Mouser is a tiny but mighty addition to any artifact-based strategy, combining efficiency with lasting value. This one-mana Cat artifact creature features both Lifelink and the Modular mechanic, ensuring it contributes to your life total before passing its strength to another machine.

Understanding Modular and Lifelink

Arcbound Mouser possesses two key abilities. Lifelink means that any damage dealt by this creature also causes you to gain that much life. The Modular 1 ability performs two functions: first, it ensures the Mouser enters the battlefield with a +1/+1 counter on it, effectively making it a 1/1. Second, when it is put into a graveyard from the battlefield, you may put all +1/+1 counters currently on it onto another target artifact creature.

Resolution Step-by-Step

When Arcbound Mouser enters the battlefield, it arrives with a +1/+1 counter already on it; this is not a triggered ability and cannot be responded to. If the Mouser deals damage, you gain life simultaneously. If the Mouser dies, its Modular triggered ability goes on the stack. You choose a target artifact creature at this time. If the target is still legal when the ability resolves, you move all +1/+1 counters that were on the Mouser onto that creature. If you control no other artifact creatures, the ability simply does nothing.

Common Mistakes to Avoid

A frequent error is forgetting that Modular can only target another artifact creature. You cannot move the counters to a non-artifact creature even if it is a Cat. Additionally, if the Mouser is exiled instead of dying (moving from the battlefield to the graveyard), the Modular trigger will not fire. Lastly, remember that Modular moves all +1/+1 counters currently on the Mouser, not just the single counter it started with, should you have added more through other spells or abilities.

Strategic Timing

Arcbound Mouser is best utilized as an early-game blocker or an aggressive attacker that forces difficult decisions for your opponent. Because it has Lifelink, it is an excellent recipient for additional equipment or counters. Ideally, you want to have another artifact creature ready on the board before the Mouser dies to ensure you don't lose the momentum of its +1/+1 counters.

Interaction examples

Scaling Your Taxes

Esper Sentinel

If you control both creatures and Arcbound Mouser dies, you can target Esper Sentinel with the Modular ability. By moving the +1/+1 counter to Esper Sentinel, its power increases. This makes its taxing ability more effective, as opponents will now have to pay more mana to prevent you from drawing cards when they cast their first noncreature spell each turn.

Boosting Utility Fliers

Ornithopter of Paradise

When Arcbound Mouser is destroyed in combat, you can target Ornithopter of Paradise with the Modular trigger. The Ornithopter, which is usually a 0/2 mana producer, becomes a 1/3 with Flying. This allows your utility creature to become a competent blocker or even a source of chip damage while still retaining its ability to tap for mana.

Guide written with AI from the official text and rulings. Official English text prevails.