I was just looking over the Tiamat stat block, and she's immune to spells of 6th level or lower.  Given that Warlocks only get spells up to 5th level and their main attack is a cantrip, they seem like they can't really contribute much to the fight.  Which is...  dumb.  

I think I may just house rule that if you've got invocations added to eldritch blast it's more than a spell and can there for effect epic level monsters like Tiamat.  

Anyone else run into this problem?  How did you solve it? 

I solve it by following the normal rules for Warlocks? They get access to higher level spells through their Mystic Arcanum Feature that do not use the spell slot resource that caps at 5th level. 

Yeah.  I get that, but that feature still only grants five castings, assuming they're all applicable to combat.  Still seems like the Warlock is at risk of getting sidelined battling the big bad.  

Quote from BMahan >>

Yeah.  I get that, but that feature still only grants five castings, assuming they're all applicable to combat.  Still seems like the Warlock is at risk of getting sidelined battling the big bad.  

 If the combat takes more that five castings, then the Warlock should have a ton of other things to do via magic items or other features during combat to assist the rest of the party. The party Wizard only has 6 spell slots over 5th level at level 20, so.....what are we talking about here? 

Yeah when looking at spells of 7th level or higher to affect her, a wizard only gets four a day. Two 7th level slots and one slot for 8th and 9th level. 

This isn't really hampering warlocks specifically so much as casters in general. God help any caster who multiclassed into a non caster class to slow their spell slot growth. While a martial with a magic weapon will be unimpeded. 

Though as noted above, spells can be used for other things like buffing the party or target any of her minions etc. It doesn't necessarily make 6th and lower level spells useless. And that's still four full rounds of combat worth of max level spells. 

A high-level warlock should also have access to numerous non-spell abilities thanks to their Eldritch Invocations and Patron abilities.

Rakshasa has the same ability (immunity to 6th level spell or lower unless it wishes to be affected by them) so you don;t really need to wait for them to be higher enough level to tackle Tiamat if you want to mess with a warlock a little bit. Putting them up against a Rakshasa could also be a good introduction to the ability so they can have some practice and test out alternate options.

Edit: and you also have Helmed Horrors with their limited spell immunity if you want to mess with them even earlier.

Tiamat said; "Warlocks taste like green licorice." ;p

Thanks for the feedback, everyone!  Clearly I don't have enough experience with high level play.

Few people ever do, as it's the rarest tier of play.

Those who are touting the Mystic Arcanum spells and Eldritch invocations are giving them more power than they have. That only gives them one spell per level above 6. That fight which I am currently in, Burns through the Mystic Arcanum spells in 4 turns if you are a level 20 Warlock. Keep in mind, depending on the campaign, your warlock will have already burned through some of the Mystic Arcanum spells. And if you happened to have taken any spells like Arcane Gate & Forcecage you aren't doing any damage. My DM countered this ineffectiveness by giving me a feat with Crown of Stars.I'm here to tell you... Warlocks against Tiamat suck.

Remember back in earlier editions where even spellcasters used actual weapons? Nothing stopping the Warlock from using a weapon if they are out of spells of the appropriate level, and as has already been pointed out, no spellcaster gets a lot of 7th-9th level slots.

The stat block for Tiamat is very old; Aspect of Tiamat lacks that immunity, though in many other ways it's more durable.

Tiamat's 5E stats might be old, but that doesn't mean they can't be used.