Actually elder brains in 5E are kinda weak. Larian invented a lot of bs for BG3 but by rules elder brains are kinda defenseless and rely on mind flayers to weaken enemies so that it can even attempt to read minds or implant suggestions. On their own they cant really read minds or dominate others unless in close proximity.
Any of the giths red dragons would in most cases wipe the floor with the brain.
Well acktually...it's a Netherbrain.
Also the intelligence, telepathy, and various psionic abilities is why they are so dangerous. They can mobilize and control massive armies.
A elder brain only has 21 Int. A PC can have more.
And WotC decided it was too complicated to have long spell lists for monsters, so the psionic abilities of a brain got reduced to a short range mind blast, detect thoughts and dominate person once a day as per normal spell rules.
The brain is aware of all thinking beings in a 5 mile radius, but can only really influence them if they are in incapacitated.
There is a reason why elder brains are only challenge rating 14 in 5E.
