7. Keywords and effects
Deathknell
Units that do one last thing when they die

says "Deathknell: Draw 1." Its reminder explains it: "When I die, get the effect." A unit with has a trigger that goes off as it dies.
How it works
It's a trigger like any other. When the unit dies, its Deathknell goes on the chain, and either player can answer it before it resolves. By then the unit is already in the trash, but its Deathknell still happens.
Only dying counts: going from the table to the trash, from lethal damage or a card that kills. A unit returned to its owner's hand hasn't died, so its Deathknell doesn't go off.
An example
Your Watchful Sentry (1 ) keeps a battlefield, and your opponent attacks it with a (3 ). The Sentry deals 1, and the Skulker deals 3: the Sentry dies, and the Skulker conquers.
But the Sentry's Deathknell goes on the chain as it dies, and you draw a card. A unit with Deathknell is never a total loss.
If several of your units with Deathknell die at once, each one goes off, and you choose their order, as with any of your triggers.
Try it
Let a unit with Deathknell die in combat, and get its effect.