The player puts those cards into their hand, then shuffles
Its reminiscent of a Turntimber Symbiosis thats just kind of worse on both ends, but getting a big creature out of a land is fine late-game utility, especially when you get the land back anyway
If the targeted creature becomes an illegal target before Drag Under resolves, you wont draw a card
These include creatures such as Glaring Freshraker, Herald of Kozilek, It That Heralds the End, Snapping Voidcraw, as well as ramp spells and mana rocks such as Cultivate, Growth Spasm, Rampant Growth, Ruin in Their Wake, Arcane Signet, Commander's Sphere, Sol Ring, Fellwar Stone, and many more
A big key to the Angel's success or failure is whether it can win fights against its most natural enemies, which leaves it coming up a little short these days due to a wealth of 4/4s both on the ground and in the air but in an era full of bears few combats were a problem