5.3 Preliminary Patch Note

[Theodric]

A mechanic that has no tells and kills you outright isn't a teacher; it's just an extremely lazy method of creating difficulty.

If you want to create a mechanic with no tells, that forces you to adjust after the fact, then it absolutely should NOT be a 1 hit kill. A mechanic giving you no opportunity to adjust and learn doesn't exist to teach; it exists to be a cheesy unavoidable death for new players.

There aren't really any mechanics in the game that lack a tell. Even those without orange AoE indicators have a specific tell attached to them. Usually involving the enemy in question moving or standing a certain way. To say nothing of the fact that there's usually a cast bar.

Typically people learn through trial, error and memorisation.