Octagram

Octagrams are essential in the repertoire of Wizards, demonologists and practitioners of Magic - but not Witches. They smack too much of "jommetry" to witches. They are complex sigils drawn on the floor, often in glowing chalk, wherein the 7a points are conjoined. Within the confines of these octagrams, huge powers can be unleashed with relative safety. Demons, other Supernatural Entities and even Death can be summoned and contained. Normally.

Others have occasionally been forced into octagrams - Albert when he was trying to defeat Death, Mort and Susan Sto-Helit when they have been Doing the Duty for Death, an Auditor when they were trying to eradicate Death, and even Rincewind once, summoned into Eric's octagram.

He then found to his cost what many demons have found through the centuries: that curse and flail as you may, the power of an octagram is too great to supersede: trying to cross the boundaries sends a powerful shock through the attempter. Death alone is seemingly immune. He can't not be summoned, but he does sometimes disconcert the wizards by appearing outside the octagram, as if proving a point. After all, some day he will summon everyone with even less chance of resistance.

See also Magic Circles. And here for more about octagrams on Roundworld.

Annotation, or at least an interesting coincidence

In the second film of the Lord of the Rings trilogy, The Two Towers, the wizard Saruman is seen to keep his farseeing crystal, the palantir, confined inside an ornate ceremonial octagram. A shout-out to the Discworld?