The Whisperer is a magic fight where the real threat comes from the floor. The tentacles are telegraphed, and reading them is the whole encounter.

Getting there

Two ways in:

Teleport nexus. Find The Whisperer in the boss teleport list. The nexus shows its difficulty rating, its notable drops and any requirements before you commit.
Command. ::whisp takes you straight there.

The command puts you in your own private instance and does not cost an instance token. There is no reason to fight this one in a public area.

The nexus lists The Whisperer as Master difficulty with no entry requirements, so nothing gates you but your own gear.

Magic attacks

The Whisperer's standard attack is magic. Pray Protect from Magic and keep it up. This part of the fight is straightforward.

This is what kills people.

Tentacles spawn four tiles diagonally from wherever you are standing, then sweep toward you. Anyone caught takes 22 to 40 damage.

Because they spawn relative to your position rather than at fixed points, you cannot memorise safe tiles. You have to react to each spawn.

Moving directly away often keeps you in its path, since it travels toward where you were. Stepping sideways clears it faster.

In a group, tentacles spawn relative to each player, so multiple sweeps cross the arena at once. Spreading out gives everyone room to dodge.

Ranged or melee, whichever you can keep damage up with while stepping constantly.
Protect from Magic. On for the entire fight.
Food. Tentacles hit up to 40 and you will not dodge every one while learning.
Open ground. Corners are death here.