The clue for 4 Across has been updated.
The Halloween Town demonspawn known as “downworlders” are trying to avoid running into a cross, so they have appointed substitutes to appear in their stead.

Across

1.Conveyance you can hire to sail against the wind and ocean’s noise
4.חצי ראש על בירה!
8.Prophet in deal gone wrong
12.Straighten the spine of viscount, wistfully
13.Thermodynamic quantity of tungsten or potassium
16.“Out ahead” is where to find the location of 19 A
17.You and I, going by foot, make something that looms consume
18.Three emerging from crypt reversed course to scare people with invented threats
19.City in the “French Hawaii”
20.Like moonwalks, spacewalks commit to long-term storage
22.Taking the mound for the first inning but leaving before the fifth, a marquee player—for example . . .
24. . . . Dr. K—falls behind western Rangers et al.
25.“Ye Olde” glyph had, at first, been a letter in Middle English
26.Messes up things to keep body parts warm
27.Moved swiftly to camouflage eavesdropping
28.They’re easily taken from outer Somalia, and one more thing
29.Japanese city is satisfied with adopted plaything, in retrospect
38.Selection from Berlioz awakens conductor
39.Dentist follows old line at the Preakness
40.Nacho ingredient: half a smoked jalapeño
41.Pay careful attention to hunk at gym
42.Quinnipiac University’s academic dormitories, originally a specific area on campus
43.Weeds, bananas, and rutabaga
45.I, returning, met a man with his paramour
46.Gun for shooters in Brooklyn evincing recoil
48.West Side Story character Krupke’s last dance move
49.Passed by, unexpectedly pleased
50.Hey, buddy, nice stone
51.Spot for a pot made from California iron
52.Region of O3 giving off oxygen
53.Welsh city using transposition awes NSA
54.Strongly impel a spinner; push without gravity
55.Gnaw at lower extremity and joint
56.10 embracing 10 on the way back to MIT dorm, colloquially

Down

1.Choose Megatron to be transformed into solenoid
2.Jack, a VI-1 alum, to use the doorbell as a way to show disrespect
3.A garment for supporting actor and winner of an Oscar in 1985
4.Fascinated by sprung trap
5.Huckleberry—one from Northern Europe
6.Mowing results in chaos when large Japanese men grab each other
7.Decapitated Iditarod participant is eponym for Poe’s house
9.Bugler’s signal when fight is given up
10.Become a fountain of tears when fight lost, right?
11.Empathetic groom (not bachelor), fathers, and the People of Israel uncommonly dined
14.Hustle, bustle, and have both a tenant and a landlord
15.Skill, I agree, involves more than a bit of curiosity
21.Give up most of the way through trivia contest, at bottom of bracket
22.Rave wildly and swear
23.#1 baseball card publisher eliminating baseball’s position #1
31.Yank is an architect at church
32.Formerly about the ear, from another culture
33.Revealed in the mirror: undressed, undressed, undressed
34.Southern antique auctioned off
35.Point where there’s no vibration (up-and-down motion) of the head and tail of animalcule
36.Everything in New York is bad
37.Fictional MIT alum found in lair on Manhattan
42.Senses using one’s ears, nose
44.Upwardly mobile poet loses excellent prop
45.Howls at the moon in Massachusetts and San Francisco, e.g.
46.Running the gamut near Australia
47.Sister captures boy’s heart with part of speech