I'm not familiar with Rolemaster, but based on what you've given us, I can say that a Warlock with the Otherworldly Patron of The Great Old One is what you are looking for.
Some benefits of this choice:
- Charisma is your primary spell-casting ability, so you'll need lots of it to be an effective warlock, which allows you better rolls for Intimidation, Persuasion, and the like.
- Being a follower of a Great Old One permits you, at level 1, to speak into the minds of anyone you can see within 30 feet. As long as they know any language, they understand you.
- Being a follower of a Great Old One expands your spell list to
include spells like detect thoughts, dominate person, telekinesis,
dissonant whispers, and other similarly "horrifying" spells (page 110
in the PHB)
- At 10th level, your thoughts cant be read unless you allow it.
- At 14th level, you can charm a single humanoid, making them your
Thrall. You can communicate telepathically with them as long as
you're on the same plane of existence, which allows you to mess with
his mind no matter what, if you desire.
- As a Warlock you have access to powerful Invocations that will help
you be a master of deception and mind-affecting spells, such as
Beguiling Influence, Mask of Many Faces, Whispers of the Grave
- The Warlock spell list has many mind-affecting spells, such as Charm
Person, Crown of Madness, Fear, Hold Person, and others.
- You have, until level 8, 2 spell slots to cast with. Your spell slots
are powerful, but limited. You will need to use cantrips for most of
the fights you're in. This also means that role-playing spells like
Contact Other Plane don't get the benefit of being casted at a higher level
and has limited casts. You get your slots back on a short rest (other
classes require long rests)
- You aren't as good as bard at inspiring your party via your mind
- Your spells known is also limited. You can get 3 additional cantrips to cast by picking the Pact of the Tome feature, and you can use the Book of Ancient Secrets invocation to get two 1st-level ritual spells, as well as the ability to inscribe more ritual spells into the book. Your limited number of spells known is probably your biggest drawback and you may be conflicted about it. Do you grab combat heavy spells to be a powerhouse in damage or control, or do you grab the RP heavy spells to be a master in between fights? (see the last header about you and your DM). Fortunately, Witch Bolt, Hex, and Eldritch Blast will do you just right for single target damage, and Hunger of Hadar and Mire of the Mind (invocation that grants Slow) can let you control large groups of enemies for your party, leaving your remaining spells known to be used for other purposes.
- You only get 1 spell known of 6th, 7th, 8th, and 9th level spells, and the selection is rather small. These act as their own slots though, so they won't particularly interfere with your ability to cast other spells between short rests. This is unique to the warlock. Some of these spells will allow you to be a necromancer type, but without the ability to cast any of them at higher levels. For example, Create Undead will always be cast at level 6.
Aren't Warlocks evil?
DnD 5e doesn't have alignment restrictions for classes. So you can be a lawful good Warlock if you can justify it. However, most Great Old One's are neutral or evil. But you can still follow an evil god without being evil. In fact, the PHB even states that
The Great Old One might be unaware of your existence or entirely indifferent to you... (pg 109)
So if you have an aversion to being evil, fear not! However, depending on your DM or your campaign, Necromancy can be seen as particularly evil. This is really only true when you're disturbing dead bodies, and not when you cast pure necromancy spells such as False Life. Spells are not inherently evil, but the circumstances around them can be. Strictly speaking, almost everyone in the DnD world (and real life) see disturbing the dead as evil, straight up. However, you might be able to justify it to your DM. Good luck doing that in a party with a Lawful Good Paladin.
Also, there is no Necromancer class either. You can be a Wizard of the School of Necromancy, but that's another question and answer altogether.
How can I enjoy being a Mentalist?
That's a pretty bold question, one that I don't think I can answer objectively. Suffice it to say that a Warlock of a Great Old One is built around mind-manipulation and madness. One of the suggested gods you follow is even Cthulhu, straight from H.P. Lovecraft's Cthulhu mythos, which is the epitome of "Mind-F***". This warlock has all the tools, it is up to you and your DM to make it enjoyable. If you're playing a heavy combat campaign, you might not find as many opportunities to use Detect Thoughts or Contact Other Plane as a more role-play heavy campaign. Either way, however, I can say from personal experience that being able to relay information to the party with one-way telepathy and Devil's Sight is very handy in a combat heavy campaign.