When you read a scroll of web, you are repeatedly given a getpos prompt for where you would like to create a web. The area in which you can create webs is identical to the scroll of stinking cloud in terms of line of sight and radius. Each position you select has a web placed on it (though if will fail if a web can’t be placed there for whatever reason, such as another trap already existing there). If you create a web on a square containing a monster, it is immediately entangled in the web.

The amount of webs you can place is 2 if the scroll is cursed, 6 if it is uncursed, and 10 if it is blessed.

The scroll and spellbook of web allow you to fire a web projectile (which would work similarly to a venom projectile, probably also being in VENOM_CLASS) or beam in a direction. If it hits a wall, it creates a web on the last non-wall space; if it hits a monster it creates a web on the monster’s space and immediately entangles it. If it goes far enough without hitting anything, it creates a web at the end of its range.

Giant spiders should possibly also be able to shoot a web like this, and if so the player should be able to do it via #monster when polymorphed into one.