Sentinel Sarah Lyons doesn't have to have been on the battlefield at the point in the turn where an artifact entered the battlefield under your control
Whether it be making tons of Squirrel tokens, utilizing your Squirrel creatures for other uses, or turning your Squirrels into an overwhelming force, theres a heavy focus on the creature type
This card can also be your commander, giving you the chance to use it multiple times over the course of a commander game
There's a hilarious game where pro player Brian Braun-Duin was playing Nadu at a Nerd Rage Games event and presented the combo, but his opponent was just like, "All right, combo off and kill me." And BBD proceeded to spend the next hour seemingly aimlessly looping through his deck
Based on the latest market prices tracked on TCG Stacked, Emry, Lurker of the Loch is worth about $2.15 USD for a regular copy and $2.89 USD for a foil copy