diff --git a/src/pyhf/experimental/modifiers.py b/src/pyhf/experimental/modifiers.py
new file mode 100644
index 0000000000..39eefaf6ca
--- /dev/null
+++ b/src/pyhf/experimental/modifiers.py
@@ -0,0 +1,167 @@
+from __future__ import annotations
+
+import logging
+from typing import Any, Callable, Sequence
+
+import pyhf
+from pyhf import events, get_backend
+from pyhf.parameters import ParamViewer
+
+import numpy as np
+
+log = logging.getLogger(__name__)
+
+__all__ = ["add_custom_modifier"]
+
+
+def __dir__():
+    return __all__
+
+
+try:
+    import numexpr as ne
+except ModuleNotFoundError:
+    log.error(
+        "\nInstallation of the experimental extra is required to use pyhf.experimental.modifiers"
+        + "\nPlease install with: python -m pip install 'pyhf[experimental]'\n",
+        exc_info=True,
+    )
+    raise
+
+
+class BaseApplier:
+    ...
+
+
+class BaseBuilder:
+    ...
+
+
+def add(funcname, par_names, newparams, input_set=None, namespace=None):
+    namespace = namespace or {}
+
+    def make_func(
+        expression: str, namespace=namespace
+    ) -> Callable[[Sequence[float]], Any]:
+        def func(deps: Sequence[float]) -> Any:
+            if expression in namespace:
+                parvals = dict(zip(par_names, deps))
+                return namespace[expression](parvals)()
+            return ne.evaluate(
+                expression, local_dict=dict(zip(par_names, deps), **namespace)
+            )
+
+        return func
+
+    def _allocate_new_param(p):
+        param_dict = {
+            'paramset_type': p['paramset_type']
+            if 'paramset_type' in p.keys()
+            else 'unconstrained',
+            'n_parameters': 1,
+            'is_shared': True,
+            'inits': p['inits'],
+            'bounds': p['bounds'],
+            'is_scalar': True,
+            'fixed': False,
+            'auxdata': p['auxdata'] if 'auxdata' in p.keys() else (0.0,),
+        }
+        return param_dict
+
+    class _builder:
+        is_shared = True
+
+        def __init__(self, config):
+            self.builder_data = {'funcs': {}}
+            self.config = config
+            self.required_parsets = {}
+
+        def collect(self, thismod, nom):
+            maskval = True if thismod else False
+            mask = [maskval] * len(nom)
+            return {'mask': mask}
+
+        def append(self, key, channel, sample, thismod, defined_samp):
+            self.builder_data.setdefault(key, {}).setdefault(sample, {}).setdefault(
+                'data', {'mask': []}
+            )
+            nom = (
+                defined_samp['data']
+                if defined_samp
+                else [0.0] * self.config.channel_nbins[channel]
+            )
+            moddata = self.collect(thismod, nom)
+            self.builder_data[key][sample]['data']['mask'] += moddata['mask']
+            if thismod:
+                if thismod['name'] != funcname:
+                    self.builder_data['funcs'].setdefault(
+                        thismod['name'], thismod['data']['expr']
+                    )
+                self.required_parsets = {
+                    k: [_allocate_new_param(v)] for k, v in newparams.items()
+                }
+
+        def finalize(self):
+            return self.builder_data
+
+    class _applier:
+        name = funcname
+        op_code = 'addition'
+
+        def __init__(self, modifiers, pdfconfig, builder_data, batch_size=None):
+            self.funcs = [make_func(f) for f in builder_data['funcs'].values()]
+
+            self.batch_size = batch_size
+            pars_for_applier = par_names
+            _modnames = [f'{mtype}/{m}' for m, mtype in modifiers]
+
+            parfield_shape = (
+                (self.batch_size, pdfconfig.npars)
+                if self.batch_size
+                else (pdfconfig.npars,)
+            )
+            self.param_viewer = ParamViewer(
+                parfield_shape, pdfconfig.par_map, pars_for_applier
+            )
+            self._custommod_mask = [
+                [[builder_data[modname][s]['data']['mask']] for s in pdfconfig.samples]
+                for modname in _modnames
+            ]
+            self._precompute()
+            events.subscribe('tensorlib_changed')(self._precompute)
+
+        def _precompute(self):
+            tensorlib, _ = get_backend()
+            if not self.param_viewer.index_selection:
+                return
+            self.custommod_mask = tensorlib.tile(
+                tensorlib.astensor(self._custommod_mask),
+                (1, 1, self.batch_size or 1, 1),
+            )
+            self.custommod_mask_bool = tensorlib.astensor(
+                self.custommod_mask, dtype="bool"
+            )
+            self.custommod_default = tensorlib.zeros(self.custommod_mask.shape)
+
+        def apply(self, pars):
+            """
+            Returns:
+                modification tensor: Shape (n_modifiers, n_global_samples, n_alphas, n_global_bin)
+            """
+            if not self.param_viewer.index_selection:
+                return
+            tensorlib, _ = get_backend()
+            deps = self.param_viewer.get(pars)
+            out = tensorlib.astensor([f(deps) for f in self.funcs])
+            results = np.zeros_like(self.custommod_mask)
+            np.place(results, self.custommod_mask, out)
+            results = tensorlib.where(
+                self.custommod_mask_bool, results, self.custommod_default
+            )
+            return results
+
+    modifier_set = {_applier.name: (_builder, _applier)}
+    modifier_set.update(
+        **(input_set if input_set is not None else pyhf.modifiers.histfactory_set)
+    )
+    return modifier_set