Min/Max Boards

And lastly, my biggest gripe with 5e: they carried on what I think was the absolutely worst idea from 4e: PCs are not part if the world. They operate by a specific set of rules that are vastly different from 'monster' rules. For example, as a PC Fighter you can't by the rules face off against an Orc fighter of similar ability. Your fighter abilities are PC only.

As somebody who hasn't yet played 5e if what you say is true I have serious issues with 5e now.  Though if I were to DM I'd probably find a way to put the PC's against NPC's using PC classes and really shake things up, because I fundamentally think that this difference is a poor design choice.  Is there some sort of lore within the system that justifies the difference in PC's versus monsters?

Maybe it's just because I haven't had a legitimate D&D session in a while, but that sounds like a fantastic session.

However, mistakes were made by both sides which may have left a sour feeling in your gut at the end of the session.  The players got cocky and tried to take on a foe based on their own meta-knowledge of the game.  If they were truly in character then their spells suddenly not working and disintegration beams flying would be some of the most terrifying things they could have experienced and they should have ran.  Instead, they made the rash and cocky decision that they, as players, can take on this encounter. 

As a DM there are several easy counters to this meta-knowledge.  You could have made the players roll dungeoneering knowledge checks to try to determine the aberrant nature of beholders and determine what they actually knew and forced them to think along the lines of 'what would my character ACTUALLY know'.  Or you could have made them roll wisdom checks and pretty much told them that this is a futile encounter that they're likely to die in based on the situation.  Either way, this whole situation should have been avoidable through this kind of player-nudging.

The second mistake was letting the players try to math things.  As a DM I regularly shut down rules lawyering by simply determining yes or no and enforcing with the players that it's chosen whether or not the rules or geometry actually say it won't work.  Because a ray has a range of only 150 feat does this mean that the ray disappears in the air after that range? if so then how come it does full damage out to that range?  Honestly if somebody presented me with this logic I'd let it work beyond its effective range, but the numbers would be reduced to account for the changes.

The third mistake I will see from time to time is that the monster was trying to kite the players.  Honestly if I were that beholder I would have ran away after determining that I wasn't going to win the fight.  The beholder's motivation of 'angry animal try to kill annoying stinging warlock' could be countered by the beholder's own wisdom or intelligence scores.  If they players are still wanting to go look for the beholder at this point, they would have a chase on their hands, and potential to lose their warlock friend for good.


All in all, though, I think that the session should have been a good warning to the players that there is little messing around going on.  If you take on an encounter that you're not likely to win they you will likely perish.  The warlock has been made an example, and luckily he only lost a session or two while the party works on putting him back to normal.

So I made a list of invocations which I think could benefit you in some way. Hope some of this helps.

Suggested invocations:
Least:
Baleful utterance
   infinite shatter may not be very powerful for attacking, but it has immense purpose when dungeon delving.  Destroy that trap from afar, break into a    door, intimidate your enemies.
Devil's sight
   use in conjunction with the lesser invoc hungry darkness to make a pain field around you which you can see through but most of your enemies will not. 

Lesser:
Curse of Despair
   nothing like a standard action to bestow curse upon your enemies.  Straight forward debuff.
Beshadowed blast (essense)
   good for blinding enemy casters to prevent them from targeting you from afar, also use with chain to potentially effect multiple targets
hungry darkness
   works well with devil's sight, basically creates a painfield over an area, then you can fight your enemies while seein just fine while they can't see.
walk unseen
   you had mentioned that you're a little glass-cannon.  This will allow you to get the jump on most foes unless your DM is smart and knows to give some    enemies detect invisibility or similar.
flee the scene
   Nothing quite like unlimited dimension doors, and the afterimage that it leaves behind could buy you a round

Greater:
Chilling tentacles
   Evard's black tenticals have their uses, even against foes much stronger.
Eldritch cone
   an AOE effect for your e-blast for delivering essence debuffs that's easier to control who gets hit, I like it.
Noxious blast
   a nasty debuff to apply to target(s)

dark:
Eldritch doom
   While it's not optimal, if you're already in melee range of your enemies this will allow you to try to apply your debuffs in a large area
Retributive invisibility
   This is way better than walk unseen, and you'll be less likely to get smacked by opponents who can't detect you properly
utterdark blast
   bwahaha a DM's worst nightmare, grants target(s) of your blast 2 negative levels.  Also gives most DM's a headache when trying to calculate what this       actually affects.
word of changing
   baleful polymorph, enough said.

As for feats i suggest quicken SLA
   allows you to blast and then attack/move 3 times a day, giving you more time to do what you do best (natural attacks)

I suppose coating your enemy in contact poison is out. When they come out of it they are forced to save vs poison a dozen or so times?