Summoning trap Idea. need some help smoothing it out.

[Jgosse]

So I had a thought for a "Trap" in a wizards tower. in a room the party find a pedestal and on top of it is a crystal. for every person standing in a 60ft radius it summons 1D4 Dretch the room is 120feeet wide and 150 long and it activates as soon as they enter the area of effect. the Dretch Do not activate the Effect.
The party is 5-6 level 5 characters and Dretch are 1/4 CR . (Is there a weaker one?)

The party Can run through and by pass the Trap or they can destroy the crystal.

What do you think, is this a good Idea or no, also how hard of an encounter is it?
the party is a assassin rogue, Draconic sorcerer, bear totem barbarian, Archfey warlock, and a lore bard. though that may change.

Any questions.

[Jgosse]

to destroy the crystal ac 10. it is a large crystal sitting on a 10 foot tall pedestal. but it has resistance(half damage) to all damagetypes and 50hp. DC 18 on a dispel check meaning the bard needs a 15 to Dispel the gem.

[Keravath]

A summoning trap sounds cool. Might also be able to do it with a modified glyph of some sort.

According to https://www.aidedd.org/dnd-encounter/index.php?l=1

A party of 5 characters (level 5-6 mix) vs 13 dretch (average number summoned) should be an easy fight. The only thing that might affect that would be positioning. If the dretches start off mixed in with the party then the number of AoE spells that the party can use is reduced. Otherwise the bard could potentially shut most of the summoned creatures down with one well placed hypnotic pattern.

[Jgosse]

Planning on summoning them next to the tower. (I already built a tower out of polyclay with embedded piece of jasper)

also it is Per turn. so if they stay in the area of effect the population continues to grow.