Unstoppable Slasher either trades with something or takes half their life points
Even though much of the card is covered in all black, Mew itself is outlined in shades of pink and yellow, with more black in the negative space to give Mew some shading

Loyal Apprentice Loyal Apprentice | Illustration by Joe Slucher Commander (1) Creature (26) Spear Spewer Reckless Fireweaver Sunshot Militia Thermo-Alchemist Brimstone Trebuchet Embraal Gear-Smasher Ingenious Artillerist Lobber Crew Molten Gatekeeper Penregon Strongbull Spireside Infiltrator Storm Caller Unruly Catapult Witty Roastmaster Crimson Fleet Commodore Stirring Bard Gorehorn Minotaurs Staunch Throneguard Arcbound Slasher Kuldotha Ringleader Panicked Altisaur Pyroclastic Hellion Tiamat's Fanatics Oliphaunt Rapacious One Bone Pit Brute Sorcery (3) Kaervek's Torch Lava Burst Flame Rift Instant (18) Dynacharge Galvanic Blast Lightning Bolt Skred Heartfire Improvised Club Incinerate Ambitious Assault Burn Bright Gnawing Crescendo Outlaws' Fury Pack Attack Path of Anger's Flame Trumpet Blast Volatile Claws Welding Sparks Farideh's Fireball Fireblast Enchantment (11) Dragon Mantle Sticky Fingers Bravado Impact Tremors Madcap Skills Makeshift Munitions Granite Grip Lightning Talons Raid Bombardment Shiny Impetus Claws of Valakut Artifact (6) Darksteel Citadel Great Furnace Prying Blade Beamtown Beatstick Goldvein Pick Javelin of Lightning Land (35) Most of the time, its best to run more than one color to have greater flexibility in what you can do with a given commander

and plenty of great sideboard options
Most of the cards that make Legacy expensive are the same cards that make Modern expensive: Snapcaster Mage, Liliana of the Veil, Misty Rainforest, etc
The enduring appeal of Forretress is also reflected in its continued presence in later generations, where it maintains its status as a reliable defensive option