How can I convert a Vacuous Grimoire from D&D 3.5e to 5e?

Question

I'm converting a module from 3.5e to 5e, and it includes a Vacuous Grimoire which I'm trying to work out how to convert.

A Vacuous Grimoire in 3.5 is a book that, upon reading, requires a Will check to see if the character loses 1 Intelligence permanently, and then a second Will to check if the character loses 2 Wisdom permanently. I understand that 5e is using 'max hp' instead of stat point loss, but since max hp resets after a long rest, it feels like that's not as much of a threat to players outside of combat (such as in this situation).

How can I effectively convert a Vacuous Grimoire to 5e in a way that its effect is actually something my players fear?

Answer 1

Couple of points:

First:

Max HP do not necessarily reset after a long rest. At least, this is not intrinsic to the long rest rule itself (which only regains hit points and half your hit dice if you spent any).

It is true, however, that an effect that drains max HP will also provide you with the means by which they are regained and this is often by resting. For example, the Wraith can drain life, but also provides the means by which it is regained:

This reduction lasts until the target finishes a long rest.

Just note that this quote is from the Wraith's Life Drain ability, not the long rest rule. Other effects that cause loss of max HP may not come back with a rest, but may instead require a healing spell of some kind (Greater Restoration will typically always work) or the sacrifice of a goat under the full moon.

Second:

Stat point loss is indeed much rarer in 5th edition than previous editions, but it is not true that it has gone completely. For example, the Shadow drains Strength.

Again, like max HP loss, any effect that drains an ability score will provide some way of getting it back. For the Shadow this is when you rest:

The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

In conclusion:

You, as the DM, are free to make your item drain Max HP or ability scores as you see fit, and it is up the you to decide how that loss can be recovered. (I would recommend some way of recovering it though, as it isn't fun for a player to feel his character has been irrevocably weakened!).

Answer 2

I think you are misinterpreting how a long rest functions:

At the end of a long rest, a character regains all lost Hit Points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest.

Lost hit points does not mean lost max hit points.

What this means is if I have 20 HP and I lose 10, when I take a long rest, I go back up to 20 because that is my max. If you lose max HP, it is not regained at the end of a long rest.

If I have 20 Max HP, lose 10 hit point and 5 max hits points, when I take my long rest, I only have 15 max HP, so I can only regain 5 of the 10 that I lost because my max HP is 15.