One of my players is playing a Blood Hunter from the D&D beyond site. He wants to have his character be the Ghost Hunter based subclass and have his backstory be that he grew up in a ghost hunting family/group, who all worshiped or looked to one or two gods. What mechanically is allowed to be a holy symbol?
[RPG] allowed to be a holy symbol
Ah yes, the effects of age, I could tell you a lot about that. But I’ll try to keep focused on in-game effects.
“I’ll regret this later”
Penalties to abilities that simply make the character weaker (like those found in older versions) are frustrating for a player, and they are bland, and don't really evoke the feel of old age. These rules simply never provided any fun, or much depth. It’s no wonder they didn't come forward to 5e.
Effects that limit the endurance a character has, that come into effect after some initial exertion, evoke the effects of age more poignantly, while letting the character still “relive past glories” in brief but glorious combat.
So very tired
The rules for exhaustion in the DMG can be utilized to simulate the fatigue of advanced age, and they come pre-play-balanced for you. A venerable character (depending on age, at the discretion of the DM) may gain a level of exhaustion from a single combat, and require a short or long rest to recover from it.
I just can’t seem to rest like I used to
Speaking of rest, an older character needs more. Simplest thing would be to double the amount of time needed to attain the benefits of the rest, but it doesn't add much color to the game, if the players are simply obliged to say “OK, then we rest for 16 hours.”
I find it’s better to “nerf” the effects of the rest, granting back fewer hit dice, hit points - or even fewer spell slots, if mental ability has been affected. That allows the oldster(s) to “try to keep up” while providing a nagging reminder they are really too old for this sort of thing.
(If you ever played 4E, this was like a non-heroic NPC taking a rest. Unlike the PC’s, the NPC would not wake up in the morning fully healed of all wounds.)
Another nice surprise
I would also suggest that these or any effects of old age should come as a surprise (especially for the prematurely aged). They always do.
My (game) experience with old age effects
I’ve used these rules for adjunct NPCs - I haven’t prematurely aged a PC. The players thought they were fair and interesting rules. It accentuated the power of the (young) PC’s, without making the NPC useless. The oldster slept/rested while the characters did ancillary stuff. It might be different for a PC. I suspect a player won't be overjoyed with having to “take it easy” but it's similar to other “curses.”
Since initially answering this, I spoke with one of my players about the age rules we used. She commented the rules made her feel protective of the NPC.
Your player has the right of it, mostly
All of the following information can be found for free in the D&D 5e Source Reference Document PDF as well as on the official 5e online supplement D&D Beyond. Page references are included for the information in the SRD PDF. All of the content quoted from the SRD can also be found in the core rulebooks.
Paladins and clerics both say the following under their spellcasting class feature (SRD pgs. 16, 31)
You can use a holy Symbol (see "Equipment") as a spellcasting focus for your [cleric/paladin] Spells.
The general rules for spellcasting foci as related to material components are as follows (SRD pg. 102):
A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
In particular, the holy symbol states (SRD pg. 67):
A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic... A cleric or paladin can use a holy symbol as a spellcasting focus. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield.
These are the only rules as far as the book is concerned. Clerics and paladins can use a holy symbol as a spellcasting focus to replace the non-costly, non-consumed material components of their spells. No stipulation RAW is made that the holy symbol must be of an exactly certain quality, type, or even a match to the deity that the cleric or paladin worships. From these quotes, we can determine that:
For spellcasting, the holy symbol is only required when you need material components
The holy symbol is a spellcasting focus. Spellcasting foci by design are only used to replace non-costly, non-consumed material components. If your player is casting a spell without material components, they do not need their holy symbol at all and can cast it normally. If the player can provide the actual material component, they can also cast it normally, even without the spellcasting focus.
speaking of material components...
Component pouches can be used by anyone
The rules for component pouches are as follows (SRD pg. 67):
A component pouch is a small, watertight leather belt pouch that has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specific cost (as indicated in a spell's description).
There are no other rules for pouches, aside from those general "spellcasting foci" rules I quoted above. Any spellcaster can use a component pouch for basically the same purpose as a focus. More on that topic can be found in this question.
If the player gets a component pouch, they can use it to cast spells that they would normally use their holy symbol for. The flavor is that the character stores the actual non-costly material components in the pouch. So, as long as the character has access to the pouch, they are actively providing those material components that a spell calls for. This, as was described above, allows the character to cast spells that require material components, even without a focus.
However, non-spell channel divinity features may require holy symbols
For example, the cleric's turn undead channel divinity says (SRD pg. 16, emphasis mine):
As an action, you present your holy symbol and speak a prayer censuring the undead.
If the player has any non-spell action such as turn undead, they need their holy symbol to use it. In this case, your cleric cannot turn undead until they have a replacement holy symbol, since they have no holy symbol to present.
Additionally, these options are not spells, so the component pouch cannot be used as a substitute in these instances.
How they replace the lost symbol is up to you.
Prices for holy symbols are found on the general equipment list (SRD pg. 69), so in theory it could be as easy to replace as going to a friendly church and buying a new one. There are no rules for "blessing" a holy symbol or the like. The description of the holy symbol item does say they can be "carefully engraved or inlaid as an emblem..." so it would imply that the symbol is more complicated than "crudely carving it out of a piece of wood", but ultimately, the DM is the one who gets to make the call on if such an attempt would work or not.
The rules text on holy symbols is pretty brief:
There are a few different options listed in the rules - amulet, emblem, and reliquary - but they only differ in weight (1, 0, and 2 pounds respectively). They all have the same cost. Exactly what they are (beyond the regular English definition of the words) is not defined - it could be steel, gold, stone, etc. As long as it's worth 5 GP and has the appropriate symbology, it's a holy symbol.
The nature of a holy symbol is not a matter for the rules, because there are no mechanics behind it. Even within the same church, followers of a given deity may use different holy symbols. The themes and design of the symbols follow a pattern (in the Realms, Kelemvor uses a scale) but the exact construction varies.
In fact, the price is only an issue for buying something that qualifies for the game mechanical use of a holy symbol, which is only relevant to Clerics and Paladins. For a lay worshipper, a token of bone or wood with the right markings may just be good enough.