The reckless slaughter of thousands of defenseless Germanic people (the tribe of the Marsi; then located in today's Northern Germany) while celebrating their feast dedicated to their goddess Tanfana in 14 A.D.

Roman soldiers under the leadership of Germanicus sneaked to their villages and killed everyone, women and men regardless of any age. Since all Marsi were drunk or asleep there wasn´t any defense. After that, the Roman intruders destroyed the sanctuary of Tanfana completely.

I´ll let Tacitus, the Roman historian, speak for itself.

"But the Roman general (Germanicus) in a forced march,
cut through the Caesian forest and the barrier which had been begun
by Tiberius, and pitched his camp on this barrier, his front and rear
being defended by intrenchments, his flanks by timber barricades.

He then penetrated some forest passes but little known, and, as there
were two routes, he deliberated whether he should pursue the short
and ordinary route, or that which was more difficult unexplored, and
consequently unguarded by the enemy. He chose the longer way, and
hurried on every remaining preparation, for his scouts had brought
word that among the Marsi it was a night of festivity, with games,
and one of their grand banquets.
Caecina had orders to advance with
some light cohorts, and to clear away any obstructions from the woods.

The legions followed at a moderate interval. They were helped by a
night of bright starlight, reached the villages of the Marsi, and
threw their pickets round the enemy, who even then were stretched
on beds or at their tables, without the least fear, or any sentries
before their camp, so complete was their carelessness and disorder;
and of war indeed there was no apprehension. Peace it certainly was
not- merely the languid and heedless ease of half-intoxicated people.

Germanicus, to spread devastation widely, divided his eager legions into
four columns, and ravaged a space of fifty miles with fire and sword.
Neither sex nor age moved his compassion. Everything, sacred or profane,
the temple too of Tamfana, as they called it, the special resort of
all those tribes, was levelled to the ground. There was not a wound
among our soldiers, who cut down a half-asleep, an unarmed, or a straggling

The Bructeri, Tubantes, and Usipetes (Germanic tribes), were roused by this slaughter,
and they beset the forest passes through which the army had to return.
The general knew this, and he marched, prepared both to advance and to fight...

Original quotation taken from "Tacitus - The Annals"

Let´s not forget about their memento.

More about it here (in German):