>>95975682 (OP)Since you haven't specified a setting or system, I'm just going to assume the 'general notion' of Liches is what you're referring to. Liches represent one of the logical end states of studious magical practitioners; on a long enough timescale Wizards or similar practitioners whose expertise is based largely on a progression of time become far more perilous to deal with. The more time a Wizard has to work the larger their personal library of spells, their stockpile of magical items, their personal army of apprentices and/or magical servitors and the more potential for them to have taken over a region or made a demiplane and twisted it to their purposes.
Becoming a Lich removes the messy variables of a mortal lifespan and also reduces the chances of being murdered. Becoming Undead often also makes you moreso a 'fixed entity' in narrative terms; it's a common enough trope that Undead become incapable of making meaningful changes to their personalities which can be either a boon or curse depending on your personal views. Senility isn't something I'd ever ascribe to any form of Undead as a possibility, though. Maybe if you're playing a Lich from a specific setting where Undead do suffer from the progression of time or it's a possibility to do your ritual ascension to Lichdom and fuck it up.