Sage Mode

Gather natural energy and enter Sage Mode. While it's on, your body gets attribute buffs and every jutsu you cast hits harder.

Controls

Hold K to gather natural energy. You can't move while you gather, and it fills your Sage bar.
Tap K to turn Sage Mode on or off.
Pick your Sage Mode variant in the Modes menu.
Your Senjutsu stat powers your Sage Mode.

Get a Sage Trial Crate. The boss Nanzen drops them, and they're also handed out in events and as staff rewards. Once a crate gives you a token, Nanzen stops dropping them for you (empty crates don't count).
Open it. It's a gamble: 70% nothing, otherwise one of three Sage Trial Tokens (Toad, Snake or Slug, 10% each).
Right-click the token in the Story Mode world to start its trial. A HUD waypoint guides you there from anywhere.

Trial	Where	What you do
Toad	Mount Myoboku	Reach 3 checkpoints and meditate (M) at each until its bar fills. Small Gamabunta waves try to hit you, and a hit resets that checkpoint. Stepping out only pauses it.
Snake	Ryuchi Cave	Walk into the point and survive the venom drain plus a wave of hostile mobs every 10s.
Slug	Shikkotsu Forest	Same as Snake, but with acid.

Leaving: wandering more than 20 blocks away cancels the trial at no cost.
Dying: costs one of the token's 3 attempts. The token itself isn't lost.
Waiting: there's a 4-hour wait between attempts.
Medical: your Medical stat lowers the venom and acid drain.

Each Sage Mode opens its family's summon contract (see Summoning):

Any Sage Mode (Toad, Snake or Slug) is needed for the True Several Thousand Hands and Light Fang questlines (see Quests).