Atiesh, Greatstaff of the Guardian

This rod is a horrible abomination on the order of life. A staff built with the unnatural eldritch magic you can cast eldritch blast at will as though you were a 20th level character

You can expend one charge to cast eldritch blast with all the Warlock eldritch invocations that have a prerequisite of eldritch blast

You can expend 1 charge to gain the effect of any eldritch invocation for 1 hour if no other duration is specified

This item functions as a pact of the chain weapon and as a pact of the tome book of shadows

If a non warlock tries to attune to this item they must succeed on a DC25 wisdom saving throw or be given 3 forms of indefinite madness as listed in the Dungeon Master's Guide. Any creature who tries to remove or suppress this madness must succeed on a DC20 wisdom saving throw or have the spell fail and be inflicted with the same madness.  

Notes: Requires Attunement by a warlock