For the endless spells, they are all pretty low cast value spells that have interesting abilities and some drawbacks
the Aracknocardron has a +1 casting/unbinding for the wizard that summons it plus it then knows all the spell from the Moonclan lore, the thing is always 1'' away from the caster and every hero phase it must do d3 mortal wounds to any one unit within 3'' of the caster, if no unit is in range the caster suffer d3 damage.
Mork's Mushroom seems like a stationary spell that affect every model in a unit ( including your own ) in 8'' of the mushroom, roll a dice every model and on a 5+ it does a mortal wounds.
The Malevolent Moon is a predatory spell that does D3 mortal wounds on a 2+ for every unit it passes though, in addition it subtracts 1 casting rolls of wizards within 8'' and 2 within 3''.
Scuttletide is pretty much like quicksilver swords you pick a unit and roll 6 dice and on a 5 up you do a mortal wound, However in addition it does damage it damage effect every time a unit moves or charge within 6 of the model. Spiderfang are immune to the damage.