Warding

Warding is an intrinsic that sometimes causes summoned monsters' melee attacks against you to fail:

X tries to attack you, but flinches away.

However, it will not prevent such monsters from casting spells or using abilities such as torment or hellfire against you. This makes it much more useful against such spells as Summon Horrible Things and Haunt than against Summon Greater Demon, for example.

Warding has no effect on monsters that are not summoned.

Sources

Warding can be obtained by wearing the amulet of warding (which also grants one level of life protection), by wielding a staff of summoning, or from various artefacts.