If you're going Warlock, then the Mask of Many Faces invocation (disguise self at will) is an alternative to changeling. Having Pact of the Chain could help with espionage, since you can get an invisible familiar (that's also thematically appropriate). A magic item that lets you cast detect thoughts would also be useful for blending in, or gift of gab for when you say something wrong. Suggestion can be useful and is on the Warlock spell list. The Whispers of the Grave invocation lets you cast speak with dead, to let you learn a bit about the dead guy postmortem. One with Shadows is just good for espionage.

The money to buy magic items is pointless, because almost no magic items have a published cost.

Magic items have ranges of cost based on rarity. Cost is halved if they are consumable. If a DM were to say "choose magic items worth total of 5,000 gp" and then left you alone, you would probably assume average price for rarity.

Another tricky part is publicly performing within expectations a 5th lvl PC, while being an effectively built 15th lvl character. You might want to avoid going full Warlock with this in mind - normal spellcasting would easily let you just expend your lower-level spells in public and use the higher-level ones for secret purposes. Pact Magic means 2 out of 3 short-rest spells are probably used in public, leaving little but the 3 Mystic Arcanums.

Changeling is a good idea - Detect Magic is likely to get thrown around at some point, and registering with an illusion-aura would need a good excuse. Svirfneblin are good if you can't rely on Plane Shift or such, assuming that non-detection covers Detect Magic. Disguise self then works, and you have excellent support for your later escape.

Skill Expert towards deception and Actor are obvioulsy feats worth considering. Since you're getting ~2 more feats than your persona, don't stack many flashy ones on top of each other. This is less likely to be an issue as a caster - but if you were a non-human with GWM, Sentinel and Polearm Master, the party might get curious at some point.

Telepathic gets you Detect thoughts, and is worth considering. (It also lets you fake being the voice of the Cult's deity heard in one's head, which could be great fun both directed at NPCs and PCs).

Metamagic Expert lets you use Subtle Spell to sneak in two secret spells.

Modify memory is an excellent warlock-spell here. The 8th level arcanum has a lot of competition, but Glibness is worth considering for this specific scenario. Dimension door is the poor man's teleport - enough to get you away from the party, but you'll need a way to maintain the distance. Obviously stuff like invisibility can be useful.

Level 1 spell scrolls are highly cost-effective, especially for a one-shot - Spellwrought tattoos lets you also go outside your spell-list. Expeditious Retreat and Longstrider, for instance, helps with escape if teleportation isn't an option. Winged Boots are likely the best investment you could make. Consumables like elemental gem's and dust of disapperance are pretty good value. If you're the only one with 1000 in magic items, you can always "discover" items on defeated foes, enabling the open use of them in later combat.

You should probably check with the DM whether it's possible to "downcast" cantrips. Eldritch blast with an extra beam is going to be a dead giveaway that something is going on. More subtle clues are likely to happen with any build where you roll higher than one should. A full Fiend Warlock would for instance have two extra damage dice to the fireball they shouldn't be throwing anyway to avoid suspicion. There are a lot of cases where you might wanna check in with the DM if it's possible for the true result to be secret - i.e., if you're always adding a flat number, you say the result without that number and the DM adds it.

If you can't downcast, you can't rely on cantrips (assuming true, public rolls). Since your persona only has 2 spell-slots per short-rest, if you want to contribute you'll need to use weapons. A fiend warlock 15 could be specced could have the blades pact and deal some damage in melee - and soak it up. Note that if the departed party-member is known not to be a hexblade, you'll have to work harder to sell the melee-build.

But at least it's possible, unlike with any other full caster, none of whom get extra attack by level 5. Which is a distinct advantage of the Warlock. Higher level spells like Plane Shift are  extremely desirable, but any fake lvl 5 caster would have to fake having few spell-slots. They'll find it harder to contribute, if scaling cantrips are a risk - specific features like Enchantment Wizard's hypnotic gaze becomes a lot more meaningful (btw, a subclass with a lot of relevant features).

Note, however, that the sorcerer might have an even better solution - Subtle Spell/Psionic Sorcery could let you secretly cast spells in combat. Very powerful, but tricky to make work - you can't just subtle-spell a fireball. There's few options that are impactful in combat without having an impact the party could question.

To get a bit more use of your higher level, consider how you could fake features with "magic" items.

Try to find out if your character would act selfishly or impulsively ("Oh yeah, I'm not here to save the world, I'm here for the loot" said ironically while detecting thoughts). If either is a possibility, you could try to pretend that you're finding and using magic items like scrolls. "Look, this is so cool - I found a scroll of summon Aberration. So say hello to Xrsghtch, she's here to help."

 While your extra 10 levels should obviously strengthen your capacity to manipulate and escape the party, take into consideration that while your party thinks they're all lvl 5 your DM has specifically created a scenario where they're not. It's quite likely that extra challenges will be thrown on them, putting pressure on you to do things your persona couldn't. Consider how your character can handle that.

Of course, you won't necessarily have to escape the party. You could aim at disposing of a weakened party before departing.

But, since the DM has given you secret information, consider that other players might also know things you don't know they don't. Maybe everyone is a secret 15th level champion of different deities. Maybe they have a secret suspicion of an imposter, but don't know who. Be careful.

Be also careful of how you play considering that they don't know. Some would not find being suddenly stabbed in the back particularly fun - consider your co-players, and try to keep things fun for everyone.

At the start of the previous post, I mentioned that going full Warlock creates a problem. Let's consider alternatives then.

Note that in most ways, higher levels in a class entails being able to fake the lower level. One issue is higher bonuses being a problem - Rogue might be excellent for passing checks when deceiving and escaping - and ironically being the one case where Assassin's higher-level features come into play - but don't really work with the massive sneak-attack damage you'll be doing.

Another, however, is unique subclass features. If the character you're imposing has a publicly known subclass independent of yours, you should avoid classes with distinguished low-level subclasses. Artificer could be surprisingly fun for this, but there's no way a Battle Smith can pass as an Artillerist. The Warlock would be rather risky if this is the case. I suspect it's not, but you should check with the DM.

If it is, though, fortunately some of the safest options synergize excellently with the warlock.

Sorcerer subclasses are favourably indistinct. Wild Magic just luckily never triggers that session. Speak Primordial, and Storm is just neglecting to use the flight. Shadow  just needs you to have Darkness learned (Devil's sight is a wise addition, though), and avoid being brought down with your enormous HP. Draconic resilience's  high AC and HP are obscured to other players. Divine soul, Clockwork soul, and Aberrant soul just happen to never see their spells used, in favour of other spells, with their other effect being minor.

Paladins only get oath spells, and channel divinity. If you forget using it, and use Harness Divine Power, people are unlikely to notice. Many of them can likely be faked - oh no, I tried turning this monster, but it saved :(

Paladin also gets oaths, of course, which excludes most of the subclasses. Vengeance and Oathbreaker still, works - as does my favourite consideration for this, Oath of the Crown. There's no conflict in the oath with what you ought to be doing - just be careful only to make promises you intend to keep. There is, however, a lot of resonance with being devoted to the lawful side of Asmodeus.  Mostly, I think it could be fun to play with the Paladin's protective tools but keeping it a secret. Working with the DM to secretly buff the players, keeping them safe until you betray them. Crown paladin gets you Aura of Protection, obviously, but Divine Allegiance also lets your lvl 15 health stand in for the lvl 5 squishies - you just need an agreement with the DM for when they should communicate that the attack missed to the player while you take the damage, and and reduce the DC of saves when within your aura.

Since Improved Divine Smite improves the odds of doing "impossible" damage and you're not loosing too much, multiclassing would be good. So it works nicely with Fiend Warlock (and lets the pact go to secretly having an invisible imp familiar, instead blade pact), but probably even better with Divine Soul sorcerer.

Note that as the tragically deceased party-member would also be a paladin, they probably carried symbols of their deity. This gives your character a strong role-playing clue - it's easy to cheese through a session mostly talking about the virtue of Tyr, throwing out some prayers etc, being overly strict about morality, and falling exactly within player expectations (as long as they don't know the character to be doubting their faith or whatever).

While it's excellent for navigating the party towards the betrayal, it is challenged after it - you can't just planeshift away. You could have decent odds of defeating the party, but you'll also want something like Dimension Door and a well placed steed. (Edit: Teleportation circle with something like Dimension Door would buy you all the time you should need. And while Divine Allegiance would be fun, similar effect is provided by Warding Bond. So if you want to be effective at betrayal, don't actually go below 9 in Warlock/Sorcerer)

Crown Paladin 8/Fiend Warlock 7 is not the most optimized build. However, I think it preserves the flavour you seemed to want, and provides a fun, unique playstyle, and is more considerate towards the players you're betraying. Revealing yourself as evil has an extra oomph when you're a paladin. Having to play around the aura's means that the player's are likely to get some clues, which become obvious only in retrospect - "oh, the reason I saved against the aberration was because Koda was standing next to me, I thought my roll was a bit low". And not just plane-shifting away gives the party a fighting chance to catch up to you. It's being a good sport about being evil.

L1: Rogue: 4 skill proficiencies, & Expertise in 2 skills, + Sneak Attack when you reveal yourself by making your first attack.

L2-5: Eloquence Bard: Additional Skill Proficiency, + Expertise in Deception and Persuasion, and you can't roll less than a 10 on the die for either of those skills, meaning your cover likely won't be blown (minimum of 23 on both Deception and Persuasion if you have at least 16 Charisma (+3 for Charisma, +10 for Expertise at L15 +10 for minimum die roll result)

L6-10: that's five levels of whatever class abilities you want to use to beat the party.

L11-15: levels in the class of the character you're imitating, so you can accurately do whatever that character can do.

And if you're a Changeling, you look exactly like the character you're imitating, and you have some immunities to spells that only affect Humanoids (like Charm Person).

I would consider taking 5 levels in whatever-the-guy-you're-imposing is, and then 10 levels in something you consider fun.

For example, you might be a rogue, doing rogue things and not doing anything a level 6+ rogue can do. Then you get the artefact, and as an action don your arcane armour from your 10 levels of armorer artificer.

Not only will it make the charade far easier to maintain without people trying to help by telling you that you can't do that yet, it'll make the reveal more of a surprise, especially if another player has suspicions and might have been planning how to deal with a rogue rogue - psych, now they're a barbarian!

I would probably focus on making a character who is decent at surviving PVP but not too OP at killing, to avoid just wiping out a player before the chase. Make a bit of a tank, I would say, and give them magic items to avoid being bound or restrained. I would not pick any items that just deal huge damage, because that would be game over - and where's the fun in that!? Pick utility items which will make them think (immovable rod on the outside of the door to block it shut, for example) and trap spells like glyph of warding to make the chase fun and challenging, rather than a slug-fest (PVP is seldom long-lived, because PC's are cannons when it comes to damage output in one turn).

Good Luck!

You may want to ask yourself if anyone from your group reads this forum and if you just shot the one shot in the shot spot

Take 5 levels in the class if the guy your impersonating. Then any other class your need the skills of to complete the mission.

Your big guy would not have picked you if they didn't think you could do the job. So ask the DM about a few gifts from the big Asz. He might provide you with a few potions to modify memory of the rest of the party. You can then just tell that that what you look like now is how you have always looked. Or more than likely he could send another minion to disfigure your face if your of the same build and size as the target. Your separated from the group and attacked/cursed. This could even help explain your memory lapses.

Thanks for the help everybody! Your advice has been really helpful. We're playing tonight, so I will update on how it all goes!

Quote from bobberuchi >>

You may want to ask yourself if anyone from your group reads this forum and if you just shot the one shot in the shot spot

 I'm not worried about that, the players only know my alt account, only the DM knows about this one. I don't think anyone uses this forum!