36 cards where the set is “sld” and the collector number >= 501 and the collector number <= 536

Karn, the Great Creator (Secret Lair Drop #501)
Ugin, the Ineffable (Secret Lair Drop #502)
Gideon Blackblade (Secret Lair Drop #503)
Teyo, the Shieldmage (Secret Lair Drop #504)
The Wanderer (Secret Lair Drop #505)
Jace, Wielder of Mysteries (Secret Lair Drop #506)
Kasmina, Enigmatic Mentor (Secret Lair Drop #507)
Narset, Parter of Veils (Secret Lair Drop #508)
Davriel, Rogue Shadowmage (Secret Lair Drop #509)
Liliana, Dreadhorde General (Secret Lair Drop #510)
Ob Nixilis, the Hate-Twisted (Secret Lair Drop #511)
Chandra, Fire Artisan (Secret Lair Drop #512)
Jaya, Venerated Firemage (Secret Lair Drop #513)
Sarkhan the Masterless (Secret Lair Drop #514)
Tibalt, Rakish Instigator (Secret Lair Drop #515)
Arlinn, Voice of the Pack (Secret Lair Drop #516)
Jiang Yanggu, Wildcrafter (Secret Lair Drop #517)
Nissa, Who Shakes the World (Secret Lair Drop #518)
Vivien, Champion of the Wilds (Secret Lair Drop #519)
Ajani, the Greathearted (Secret Lair Drop #520)
Domri, Anarch of Bolas (Secret Lair Drop #521)
Nicol Bolas, Dragon-God (Secret Lair Drop #522)
Ral, Storm Conduit (Secret Lair Drop #523)
Sorin, Vengeful Bloodlord (Secret Lair Drop #524)
Tamiyo, Collector of Tales (Secret Lair Drop #525)
Teferi, Time Raveler (Secret Lair Drop #526)
Angrath, Captain of Chaos (Secret Lair Drop #527)
Ashiok, Dream Render (Secret Lair Drop #528)
Dovin, Hand of Control (Secret Lair Drop #529)
Huatli, the Sun's Heart (Secret Lair Drop #530)
Kaya, Bane of the Dead (Secret Lair Drop #531)
Kiora, Behemoth Beckoner (Secret Lair Drop #532)
Nahiri, Storm of Stone (Secret Lair Drop #533)
Saheeli, Sublime Artificer (Secret Lair Drop #534)
Samut, Tyrant Smasher (Secret Lair Drop #535)
Vraska, Swarm's Eminence (Secret Lair Drop #536)

The message was clear to Sorin—time to search for something else.